In the interwar years, memories of devastation, domestic political pressures, and deep ideological suspicion made cooperation fragile. As new crises emerged, the major powers often hesitated, acted alone, or failed to trust one another.

Collective security under strain

Interwar diplomacy depended heavily on collective security, the idea that peace could be protected if states acted together against aggression rather than leaving each country to defend itself alone.

This principle sounded strong in theory, but it required confidence, speed, and unity. In practice, the states most capable of enforcing peace were divided by different priorities. Britain and France feared another general war, the United States avoided long-term political commitments in Europe, and many western leaders distrusted the Soviet Union more than they trusted it as a partner. Because of these divisions, aggressors faced uncertainty and delay rather than a firm united response.

British and French fear of another war

The shadow of World War I

For Britain and France, the First World War was not a distant memory but a recent trauma. Millions had been killed or wounded, whole regions had been destroyed, and the social and financial costs of war remained visible. Many voters, veterans, and politicians believed that almost any compromise was preferable to another continental conflict.

This fear produced a strong climate of pacifism, especially in the 1920s and early 1930s. Public opinion often favored negotiation, disarmament, and international conferences over military threats. Leaders also worried that modern war would bring mass bombing of civilians, economic collapse, and political unrest at home. Even when politicians recognized danger, they feared that firm resistance might trigger the very war they hoped to prevent.

Strategic and economic caution

France was especially anxious about security because of its border with Germany, yet it was also demographically weaker than its eastern neighbor and worried about fighting another large war without reliable allies. Britain preferred flexible diplomacy and naval strength, but it too faced financial constraints and imperial commitments around the world. During the Great Depression, both governments were cautious about rearmament and reluctant to take actions that might require major military spending.

As a result, British and French policy often became defensive rather than assertive. Their fear did not mean they ignored aggression entirely; rather, they hesitated, delayed, and searched for negotiated settlements. That hesitation weakened deterrence and signaled that collective resistance might not be immediate or united.

American isolationism

Limits of U.S. involvement

After World War I, many Americans concluded that involvement in European power politics had been costly and disappointing. The United States remained economically important, but it did not commit itself to enforcing the postwar peace in the way many Europeans had hoped. This outlook is commonly described as isolationism.

A 1939 political cartoon by Clifford K. Berryman portraying leading supporters of the Neutrality Act. The image captures how isolationist lawmakers framed neutrality as a safeguard against being drawn into another European war. It helps explain why U.S. policy reduced the credibility of collective action during interwar crises. Source

The most important consequence was political, not simply economic: the United States stayed outside any durable system of collective enforcement. Without American participation, states considering resistance to aggression could not count on the full power of the world’s largest industrial economy. This made threats of joint action less convincing.

Effects on interwar diplomacy

American isolationism also encouraged caution in Europe. If Britain and France believed they might have to confront aggressors without U.S. backing, they became even less willing to take risks. Neutrality-minded attitudes in the United States reinforced the sense that European crises should be contained without expecting decisive American intervention. Aggressor states could therefore assume that opposition would likely be limited, slow, or fragmented.

Distrust of the Soviet Union

Ideology and diplomatic suspicion

Distrust of the Soviet Union had deep roots. The Bolshevik Revolution frightened many conservative and liberal politicians in Europe, who associated the Soviet regime with revolution, class conflict, and the destruction of existing political and social orders. Anti-communism shaped diplomacy throughout the interwar period.

Even when Soviet leaders advocated cooperation against aggressive states in the 1930s, many western officials doubted Soviet motives, questioned Soviet military effectiveness, or feared that close cooperation would strengthen communism at home. This meant that a potentially important counterweight to aggression was not fully integrated into a trusted diplomatic front.

A divided anti-aggression coalition

Because western democracies and the Soviet Union did not trust one another, the possibility of a broad coalition remained weak.

Some leaders considered communism a threat equal to, or greater than, the actions of revisionist states seeking to overturn the postwar order. That attitude made coordinated resistance much harder. Instead of presenting a clear warning that aggression would face combined opposition, the major powers remained suspicious and divided.

Why these divisions mattered

When aggressive actions tested the interwar order, opponents of aggression often responded with speeches, conferences, or limited penalties rather than united force. The problem was not simply lack of information about danger. It was the absence of shared will. Fear of war made Britain and France cautious, isolationism reduced American commitment, and distrust of the Soviet Union blocked deeper cooperation.

These conditions undermined deterrence. Aggressor governments could observe that the states defending peace were unlikely to act quickly together. Collective resistance depended on credibility, but credibility weakened when the major powers appeared unwilling to cooperate across ideological and national lines. The interwar international system therefore became increasingly vulnerable to challenge.