Invasion of the United States

The concept of an invasion of the United States relates to military theory and doctrine which address the feasibility and practicality of a foreign power attacking and successfully invading the United States of America. The United States has been physically invaded a few times, once during the War of 1812, once during the Mexican–American War, several times during the Border War, and once during World War II. During the Cold War, most of the U.S. military strategy was geared towards repelling an attack on the United States by the Soviet Union.
The military history of the United States began with a foreign power on U.S. soil: the British Army during the American Revolutionary War. Following American independence, the next occurrence of an attack on American soil was during the War of 1812, also with Britain, and also the first and only time since the end of the Revolutionary War in which a foreign power occupied the American capital.
On April 25, 1846, Mexican forces invaded Brownsville, Texas and attacked U.S. troops patrolling the Rio Grande river in an incident known as the Thornton Affair, sparking the Mexican–American War. While the Texas Campaign remained the only campaign on American soil, the rest of the action in that conflict occurred in California and New Mexico, then part of Mexico, and in current Mexico.
The American Civil War may be seen as an invasion of home territory to some extent, with both the Confederate and Union armies each making forays into the other side's home territory. After the Civil War, the threat of an invasion from a foreign power was small, and it was not until the 20th century that any real military strategy was developed to address the possibility of an attack on America.
In 1915, the Liberating Army of Races and Peoples attempted to execute its Plan of San Diego to reconquer the southwestern United States, setting off the Bandit War and conducting raids into Texas from across the Mexican Border.
On March 9, 1916, Mexican revolutionary Pancho Villa and his Villistas invaded Columbus, New Mexico in the Border War's Battle of Columbus, triggering the Pancho Villa Expedition in response, led by Major General John J. Pershing.

European threats

Until the early 20th century, the greatest potential threat to attack the United States was seen as the British Empire. To that end, military strategy was developed to not only forestall a British attack, but also attack and occupy Canada. "War Plan Red" was specifically designed to deal with a British attack on the United States and a subsequent invasion of Canada. Similar plans existed for a 20th-century war with Mexico, although the ability of the Mexican Army to attack and occupy American soil was considered negligible, as demonstrated by the Mexican reluctance to accept the provisions of the Zimmermann Telegram. Mexican rebels led by Pancho Villa did briefly invade the U.S. on supply raids during World War I.
In 1921, Canadian Lieutenant Colonel James "Buster" Sutherland Brown drafted what can be called the Canadian version of War Plan Red, Defence Scheme No. 1. According to the plan, Canada would invade the United States as quickly as possible if evidence of an American invasion was found. The Canadians would gain a foothold in the northern U.S. to allow time for Canada to prepare its war effort and receive aid from Britain. They would also destroy key bridges and railroads. The plan had detractors, who saw it as unrealistic, but also supporters who believed it could conceivably have worked.
On the opposite side of the Atlantic, Imperial German plans for the invasion of the United States were maintained from 1897 to 1906, but were not seriously considered because the German Empire had insufficient resources to carry them out successfully. Early versions planned to engage the United States Atlantic Fleet off Norfolk, Virginia, followed by shore bombardment of eastern cities. Later versions envisioned a land invasion of New York City and Boston. The foreign policy of Kaiser Wilhelm II, sought to limit the United States' ability to interfere in European affairs, rather than as a territorial conquest. Until April 6, 1917, while the U.S. was neutral during World War I, German agents were dispatched to the country to prevent supplies from being sold to the Allied Powers, culminating in sabotage operations like Black Tom and Kingsland.

World War II

During World War II, the defense of Hawaii and the continental United States was part of the Pacific theater and American theater respectively. The American Campaign Medal was awarded to military personnel who served in the continental United States in official duties, while those serving in Hawaii were awarded the Asiatic-Pacific Campaign Medal.

Nazi Germany

When war was declared between Germany and the U.S. in 1941, the German High Command immediately recognized that current German military strength would be unable to attack or invade the United States directly. Military strategy instead focused on submarine warfare, with U-boats striking American shipping in an expanded Battle of the Atlantic, particularly an all-out assault on U.S. merchant shipping during Operation Drumbeat.
Adolf Hitler dismissed the threat of America, stating that the country had no racial purity and thus no fighting strength, further quoted that "The American public is made up of Jews and Negroes". German military and economic leaders had far more realistic views, with some such as Albert Speer recognizing the enormous productive capacity of America's factories as well as the rich food supplies which could be harvested from the American heartland.
In 1942, German military leaders did briefly investigate and consider the possibility of a cross Atlantic attack against the U.S.—most cogently expressed with the RLM's Amerika Bomber trans-Atlantic range bomber design competition, first issued in the spring of 1942—proceeded forward with only five airworthy prototype aircraft created between two of the competitors, but this plan had to be abandoned due to both the lack of staging bases in the Western Hemisphere, and Germany's own rapidly decreasing capacity to produce such aircraft as the war wore on. Thereafter, Germany's greatest hope of an attack on America was to wait to see the result of that nation's war with Japan. By 1944, with U-Boat losses soaring and with the occupation of Greenland and Iceland, it was clear to the German military leaders that the dwindling German armed forces had no further hope to attack the United States directly. In the end, German military strategy was in fact geared toward surrendering to America, with many of the Eastern Front battles fought solely for the purpose of escaping the advance of the Red Army and surrendering instead to the Western Allies.
One of the only officially recognized landings of German soldiers on American soil was during Operation Pastorius, in which eight German sabotage agents were landed in the United States by U-Boats. The team was quickly captured and put on trial as spies, rather than prisoners-of-war, due to the nature of their assignment. After the court found them guilty of espionage, six German agents were executed in the electric chair at the Washington, D.C. jail. The other two were not put to death and instead received prison terms because they willingly turned on their comrades by defecting to the United States and told the FBI about the mission's plan. In 1948, three years after World War II ended, the two were freed and returned to then Allied-occupied Germany, later to be divided between West and East Germany.
The Luftwaffe began planning for possible trans-Atlantic strategic bombing missions early in World War II, with Albert Speer stating in his own post-war book, , that Adolf Hitler was fascinated with the idea of New York City in flames. Before his Machtergreifung in January 1933, Hitler had already, in 1928, thought that the United States would be the next serious foe the future Third Reich would need to confront, after the Soviet Union. The proposal by the RLM to Germany's military aviation firms for the Amerika Bomber project was issued to Reichsmarschall Hermann Göring in the late spring of 1942, about six months after the attack on Pearl Harbor, for the competition to produce such a strategic bomber design, with only Junkers and Messerschmitt each building a few airworthy prototype airframes before the war's end.

Imperial Japan

The feasibility of a full-scale invasion of Hawaii and the continental United States by Imperial Japan was considered negligible, with Japan possessing neither the manpower nor logistical ability to do so. Minoru Genda of the Imperial Japanese Navy advocated invading Hawaii after attacking Oahu on December 7, 1941, believing that Japan could use Hawaii as a base to threaten the continental United States, and perhaps as a negotiating tool for ending the war. The American public in the first months after the attack on Pearl Harbor feared a Japanese landing on Hawaii and the West Coast of the United States, eventually reacting with alarm to a rumored raid in the Battle of Los Angeles. Although the invasion of Hawaii was never considered by the Japanese military after Pearl Harbor, they did carry out Operation K, a mission on March 4, 1942, involving two Japanese aircraft dropping bombs on Honolulu to disrupt repair and salvage operations following the attack on Pearl Harbor three months earlier, which only caused minor damage.
On June 3/4, 1942, the Japanese Navy attacked Alaska as part of the Aleutian Islands Campaign with the bombing of Dutch Harbor in the city of Unalaska, inflicting destruction and killing 43 Americans. A few days later, 6,000–7,000 Japanese troops landed and occupied the Aleutian Islands of Attu and Kiska; they were driven out entirely a year later between May and August 1943 by U.S. and Canadian forces. The campaigns of Attu and Kiska in early June 1942 were the only two foreign invasions of the United States during World War II and the first significant foreign occupation of American soil since the War of 1812. Japan also conducted air attacks through the use of fire balloons. Six American civilians were killed in such attacks; Japan also launched two manned air attacks on Oregon as well as two incidents of Japanese submarines shelling the U.S. West Coast.

Cold War

During the Cold War, the primary threat of an attack on the United States was viewed to be from the Soviet Union. In such an attack, nuclear warfare was projected to almost certainly happen, mainly in the form of intercontinental ballistic missile attacks as well as Soviet Navy launches of SLBMs at U.S. coastal cities.
The first Cold War strategy against a Soviet attack on the United States was developed in 1948, made into an even firmer policy after the Soviet development of the nuclear weapon in 1949. By 1950, the United States had developed a defense plan to repel a Soviet nuclear bomber force through the use of interceptors and anti-aircraft missiles, while at the same time launching its own bomber fleet into Soviet airspace from bases in Alaska and Europe. By the end of the 1950s, both Soviet and U.S. strategy included nuclear submarines and long range nuclear missiles, both of which could strike in as little as ten to thirty minutes while bomber forces took as long as four to six hours to reach their targets. The concept thus developed of the nuclear triad where all three weapons platforms would be coordinated in unison for a devastating first strike, followed by a counterstrike, accompanied by "mopping up" missions of nuclear bombers.
Operation Washtub was a top secret joint operation between the United States Air Force and the Federal Bureau of Investigation. Primarily lead by J Edgar Hoover then his protege Joseph F Carroll, the Operation was carried out during the Cold War with the primary goal of leaving Stay Behind Agents in the then Territory of Alaska for covert intelligence gathering, and a secondary goal of maintaining a evasion and escape facilities for captures friendly American Forces.
On June 22, 1955, during the Cold War, a US Navy P2V Neptune with a crew of 11 was attacked by two Soviet Air Forces fighter aircraft along the International Date Line in international waters over the Bering Straits, between Siberia's Kamchatka Peninsula and Alaska. The P2V crashed on the island's northwest cape, near the village of Gambell. Villagers rescued the crew, 3 of whom were wounded by Soviet fire and 4 of whom were injured in the crash.
American nuclear warfare planning was nearly put to the test during the Cuban Missile Crisis. The subsequent blockade of Cuba also added a fourth element into American nuclear strategy, this being surface ships and the possibility of low yield nuclear attacks against deployed fleets. Indeed, the United States had already tested the feasibility of nuclear attacks on ships under Operation Crossroads. Reportedly, during the Cuban missile crisis, one Soviet submarine nearly launched a nuclear torpedo at an American warship, yet the three officers required to initiate the launch could not agree to do so.
By the 1970s, the concept of mutually assured destruction led to an American nuclear strategy which would remain relatively consistent until the end of the Cold War.

Modern era

In the theater of 21st century warfare, United States strategic planners have been forced to contend with various threats to the United States ranging from direct attack, terrorism, as well as unconventional warfare such as a cyber war or economic attack on American investments and financial stability.

Direct attack

Several modern-day armies operate nuclear weapons with ranges in the thousands of kilometers. The U.S. is therefore vulnerable to nuclear attack by powers such as the United Kingdom, Russia, China, France,
and India. However, the UK and France are both members of NATO and so an attack on the U.S. by either of these countries is very unlikely.
The United States Northern Command and the United States Indo-Pacific Command are the top U.S. military commands overseeing the defense of the continental United States and Hawaii respectively.

Cyber and economic attacks

The risk of cyber-attacks on civilian, government, and military computer targets was brought to light after China became suspected of using government-funded hackers to disrupt American banking systems, defense industries, telecommunication systems, power grids, utility controls, air traffic and train traffic control systems, as well as certain military systems such as C4ISR, and ballistic missile launch systems.
Attacks on the U.S. economy, such as efforts to devalue the dollar or corner trade markets to isolate the United States, are currently considered another method by which a foreign power may seek to attack the United States.

Geographic feasibility

Many experts have considered the United States to be entirely uninvadable, because of the country's major industries, reliable and fast supply lines, large geographical size, geographic location, population size, and difficult regional features. For example, the deserts in the Southwestern United States and the Great Lakes in the Midwestern United States insulate the major U.S. population centers from threats of invasion. An invasion from outside of North America would require long supply chains across the Pacific or Atlantic Oceans, for a great reduction of overall power. Notably, no nation-state of power sufficient to threaten the U.S. exists on the North American continent: Canada and Mexico generally enjoy friendly relations with the U.S. and are militarily weak in comparison.
Military expert Dylan Lehrke noted that the amphibious-assault on either the West Coast or East Coast of the United States is simply too insignificant to get a beachhead on both coasts. Even if the foreign power managed to go undetected in light of modern surveillance capability, they still could not build up a force of any size before being pushed back into the sea. In addition, Hawaii is largely protected by the 40,000-strong U.S. military with valuable assets, which acts as a huge deterrent to any foreign invasion of the island state and thereby the U.S. continent. Thus, the invasion of the U.S. continent would have to come from the land borders through Canada or Mexico. While an attack from Mexico can be possible, California and Texas have the largest concentration of defense industries and military bases in the country, providing an effective deterrent from any Mexico-based attack. An attack launched from Canada on the Midwestern or Western United States would be limited to light infantry and would fail to take over population centers or other important strategic points, due to the U.S. having national parks there either in wilderness or cold conditions. This provides U.S. military personnel or civilian militias an advantage to conduct guerrilla warfare.

In popular culture

A number of films and other related media have dealt with fictitious portrayals of an attack against the United States by a foreign power. One of the more well-known films is Red Dawn, detailing an attack against the U.S. by the Soviet Union, Cuba, and Nicaragua. A 2012 remake detailed a similar attack launched by North Korea and an ultranationalist-controlled Russia. Other U.S. invasion films include Invasion U.S.A., Olympus Has Fallen, and White House Down, as well as The Day After and By Dawn's Early Light, both of which detail nuclear war between U.S. and Soviet forces. A bloodless Soviet takeover aftermath is depicted in the 1987 miniseries Amerika. In Philip K. Dick's The Man in the High Castle, the United States is occupied by both Nazi Germany and Imperial Japan, separated by a neutral zone, after invasions of both the West Coast and the East Coast.
A terrorist occupation of Washington, D.C. was the subject of a ' cartoon episode, when Serpentor led Cobra forces to occupy the American capital. A terrorist occupation of the capital was also seen in '.
In the video game ', Russia invades several parts of the United States, including Washington, D.C., in retaliation to a supposedly U.S.-assisted terrorist attack on a Russian airport. In ', the battle spreads to New York. The video game Homefront depicts an invasion of the U.S. by a unified Korea while ' depicts North Korea invading and occupying the United States. In the real time strategy game World in Conflict, Soviet forces invade and occupy the Pacific Northwest region of the United States. In the game ' is an alternate universe of the Axis Powers winning World War II which results Nazi Germany and Japan invading the United States in 1953. Bethesda Softworks's ' and ' are set in a world where Germany has won World War II, including a mainland invasion of U.S. after a nuclear bomb hit New York City. The 2003 video game Freedom Fighters is set in an alternate history where the Soviet Union won the Cold War, conquered most of the world and has invaded the United States from both Alaska and New York City. In the video game the Soviet Union launches a massive invasion of the United States, with an emphasis on deploying psychic beacons in order to mind control the population.