Opposition to World War II was expressed by the governments and peoples of all combatant nations to various extents. Initial reluctance for conflict in the Allied democratic nations changed to overwhelming, but not complete, support once the war had been joined. Some politicians and military leaders in the Axis powers opposed starting or expanding the conflict during its course. However, the totalitarian nature of these countries limited their effect. Noncombatant nations opposed joining the war for a variety of reasons, including self preservation, economic disincentives or a belief in neutrality in upon itself. After the war the populations of the former Axis powers mostly regretted their nations' involvement. In contrast, the people of Allied nations celebrated their involvement and the perceived just nature of the war, particularly in comparison with World War I.

Following World War I the League of Nations was formed in the hope that diplomacy and a united international community of nations could prevent another global war. However, the League and the appeasement of aggressive nations during the invasions of Manchuria, Ethiopia and the annexation of Czechoslovakia was largely considered ineffective. Opposition to these invasions sometimes also came from politicians within the aggressor nations such as Japanese Minister Kijūrō Shidehara. A school of historical thought held the appeasement precipitated a wider war by emboldening aggressive nations.

Opposition to what would become World War II reached its height in the German military with the Oster conspiracy, a plot to remove Hitler from power should the pressure placed on Czechoslovakia lead to war. No similar plans are known for the invasion of Poland.

The public sentiment of interwar Poland was dominated by the idea that their nation was formed through war and could only be maintained by a willingness for future wars. Diplomatic negotiations were pursued with Germany, but fear of compromise leading to a slow loss of sovereignty, as with Czechoslovakia, led Polish leaders to put their faith in a British and French military alliance.

Throughout the British Empire pacifists were jailed for expressing antiwar sentiment. Also Oswald Mosley and his British Union of Fascists were opposed to war, believing that another world war against Germany was not in Britain's national interest and that Britons should "fight for Britain alone". Editorials and cartoons in Action often asserted that the British Empire needed to prepare for a defensive war against Japan and that war with Germany would put Britain's interests in Asia in jeopardy. Mosley devoted all of the party's efforts to the "Peace Campaign", calling for a referendum on the continuation of the war and advocating a negotiated peace treaty with Germany. The campaign ended after Mosley and many other senior BUF members were interned under Defence Regulation 18B in May 1940.

Socialists in Britain were divided in the 1930s. There was a strong element of pacifism in the socialist movement, for example in Britain's Independent Labour Party. The commitment to pacifism, however, was balanced by militant anti-fascism. During its Popular Front period, the Comintern allied with other anti-fascist parties, including right-wing parties. This policy was terminated by the Comintern when the Soviet Union signed a non-aggression pact with Adolf Hitler in August 1939.

In British India, independence leader Mahatma Gandhi led the Quit India Movement in order to obstruct any efforts to support the British in the war and to demand complete independence of India from the British rule.

In a Gallup poll conducted in the first days of the war (between September 1 and September 6, 1939), Americans were asked if the US should "declare war on Germany in support of England, France and Poland and should deploy forces to assist those countries." with 90% of respondents saying no and 8% saying yes. In a separate question from the poll, respondents were asked what level of assistance should be given to the British, Polish and French. When asked about selling food, 74% agreed while 27% disagreed; for sending airplanes "and other war materials" to the United Kingdom and France 58% would agree with 42% disagreeing; when asked if army and naval forces should be deployed "abroad" to fight Germany 16% said yes with 84% saying no.