Why was the us reluctant to respond to fascist military aggression?

Answer 1

The general public in the US and its representatives in Congress were in a large majority isolationists during the period between WW1 and WW2. They were firmly convinced that the US should not get itself involved in the affairs of other countries in Europe and Asia, let alone get itself involved in any foreign war. There was a long tradition of US isolationism, starting with George Washington's maxim that the US should keep out of Europe's wars. Then there was the general opinion that US involvement in WW 1 had cost a great number of US casualties and a huge amount of money without serving any discernable interest of the US. The attitude of the other Allies' leaders after the war was seen as mostly self-serving and severely lacking in appreciation of US sacrifices and contributions.

The US public and most of its political leaders saw the rise of fascism as a purely European issue. Much of Germany's agression in the Thirties was moreover seen as aimed only at territories that had once been part of the German Empire before WW 1 or at territories with a large German population that was asking itself to be made part of Germany.

It would take Pearl Harbour to finally change the US public's attitude towards isolationism.