The US senate rejected joining the League of nations
Most of the isolationists felt that European problems should be left to the Europeans to solve and that it was unnecessary for the US to become involved in a foreign war .
Isolationists and America Firsters
Isolationists made little effort in the early (Post Dien Bin Phu) days. It was the anti war protests against the war and the draft that fought against the Vietnam War.
Many Americans became Isolationists after World War 1 since they felt there were millions of needless deaths and the issues and troubles in Europe were not our troubles. They were right in essence but they failed to understand that World War 2 was about them.
isolationists
The attack upon Pearl Harbor, Hawaii and World War 2 were the primary reasons isolationists gave up their notions of isolationism.
Pearl Harbour made the isolationists finally realise that if the US didn't involve themselves then the world would be overcome by the Nazis and their Japanese allies.
The government leaders were furiious and hurt and sad. The Americans were shocked, hurt, angry, surprised, confused and the isolationists came to realize war can happen to us and we have to fight back and the Europeans were in trouble. When Hitler declared war on the US they finally understood the US cannot just ignore war in the world.
Isolationists thought that the US should avoid any foreign alliances. The traditional policy of the US since independence was not to get involved in foreign affairs (ie. European affairs). There was a very strong feeling in the US that American participation in World War I had been a mistake. Americans felt that the European powers remained greedy squabblers with little interest in the ideals of democracy and self-determination. It was felt that America gained nothing from the war, which had costs many soldiers' lives, and that the world was not a better place afterwards. The lesson that these isolationists drew from this was that it was futile for the US to get involved in other country's disputes, since that would only result in involving the US in faraway wars that would cost money and lives for no purpose.
to show strength
The United States went through a period of isolationism between 1937 and 1945. During this time, the isolationists tried to keep the country out of the war in Europe. They wanted the country to focus on itself not the world.
Attached is a link to a good World War 2 timeline that ranges from 1931 to the 1945 treaties.