Short answers, pulled from the story.
The League of Nations aimed to prevent global conflict through diplomacy and a united international community. The organization failed to stop aggressive nations during the invasions of Manchuria and Ethiopia.
Opposition reached its height in the German military with the Oster conspiracy which aimed to remove Hitler from power. Admiral Yamamoto argued against attacking America while Rudolf Hess traveled to Scotland in May 1941 to attempt peace negotiations.
Oswald Mosley and his British Union of Fascists opposed war believing it was not in Britain's national interest. They devoted all efforts to the Peace Campaign calling for a referendum on the continuation of the war until members were interned under Defence Regulation 18B in May 1940.
A Gallup poll conducted between September 1 and the 6th of September 1939 found that 90% of Americans said no to declaring war on Germany. Only 16% said yes to sending army and naval forces abroad while 84% said no.
Communist parties around the world reversed their stance when Germany invaded the Soviet Union on the 22nd of June 1941. They then advocated that material support be extended to the Soviets after previously following orders from Moscow.