The United States and Japan had been at war since Japanese forces bombed Pearl Harbor in December 1941. After the United States successfully detonated the world’s first atomic bomb in a July 1945 test, President Harry S. 9, the Soviet Union declared war on Japan, and then the United States dropped the bomb on Nagasaki.

Although U.S. isolationism was not the only cause of WWII it was one of the main reasons for the start of the war because it allowed authoritarian rule to sweep the world with the weakened League of Nations, contributed to the worsening of the Great Depression, and made diplomatic resolve abroad impossible.

The economic depression that ensued after the Crash of 1929 further committed the United States to doctrine of isolationism, the nation focusing instead on economic recovery. As 1940 became 1941, the actions of the Roosevelt administration made it more and more clear that the U.S. was on a course to war.

Captain Robert Moffat Losey

On 11 December 1941, four days after the Japanese attack on Pearl Harbor and the United States declaration of war against the Japanese Empire, Nazi Germany declared war against the United States, in response to what was claimed to be a series of provocations by the United States government when the U.S. was still …

Operation Pastorius The largest invasion of American soil during World War II came in the form of eight Nazi saboteurs sent to the United States on a doomed mission known as Operation Pastorius. Each team carried up to $84,000 in cash and enough explosives to wage a long campaign of sabotage.

The short answer is the invasion would be stopped cold with a massive loss of life for the invasion force. An invasion of the US is impossible with the current military assets of the ENTIRE world. The US alone, possesses the ability to project force across the globe.

Soviet Union