Why did the wartime alliance break down after 1945?

The alliance of the United States, Britain and the Soviet Union held together to defeat Germany, but once the common enemy was gone the deep differences resurfaced. There was a fundamental clash of ideology between American capitalism and democracy and Soviet communism. The wartime conferences had left disputes over the future of Germany and Eastern Europe unresolved, and each side distrusted the other's actions, the Soviets building a buffer zone of communist states and the West fearing Soviet expansion. Mutual suspicion turned former allies into Cold War rivals.

What was containment, and how did the Truman Doctrine and Marshall Plan show it?

Containment was the American policy of stopping the spread of communism beyond where it already existed. The Truman Doctrine (1947) pledged American support to countries threatened by communism, signalling the United States would actively resist Soviet expansion. The Marshall Plan (1947 onward) backed this with large-scale economic aid to rebuild Western Europe, strengthening it against communism. Together they marked the United States taking the lead in opposing the Soviet Union and deepened the division of Europe, as the Soviet bloc rejected the aid.

What was the Berlin Blockade and airlift?

In 1948 Stalin blockaded West Berlin, cutting off road and rail access to the Western-controlled sectors deep inside the Soviet zone of Germany, partly in response to the West introducing a new currency. The Western powers responded with the Berlin Airlift, flying in supplies for almost a year until Stalin lifted the blockade in 1949. The crisis was a major early confrontation that confirmed the division of Germany, with the creation of West and East Germany, and showed both sides would stand firm short of war.

Why were NATO and the Warsaw Pact formed?

The West formed NATO in 1949 as a military alliance for collective defence, so an attack on one member would be treated as an attack on all, reassuring Western Europe after the Berlin crisis. The Soviet Union and its satellites formed the Warsaw Pact in 1955, partly in response to West Germany joining NATO. The two rival military alliances completed the division of Europe into two armed camps and institutionalised the Cold War standoff.

How is the origins of the Cold War examined in O-Level Elective History?

The origins and development of the Cold War in Europe is one of the issues that can be set as the source-based case study, so you must know the breakdown of the alliance, containment, the Berlin Blockade and the rival blocs in depth and be ready to use sources on them. Structured essays also ask 'how far' a particular factor, such as ideology or specific Soviet or American actions, caused the Cold War, so reaching a balanced judgement on responsibility is essential.

SingaporeHistory

Origins of the Cold War: the breakdown of the wartime alliance, the Truman Doctrine and Marshall Plan, the Berlin Blockade and the rival alliances for O-Level Elective History

A module overview of the origins of the Cold War in Europe for Singapore O-Level Elective History. How the wartime alliance between the United States and the Soviet Union broke down, the American policy of containment through the Truman Doctrine and Marshall Plan, the Berlin Blockade and airlift, and the formation of NATO and the Warsaw Pact.

Generated by Claude Opus 4.86 min readSEAB-2174Updated 2026-06-13

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

Why this module matters
From alliance to rivalry
Crisis and the division of Europe
Judging who was responsible
Check your knowledge

Why this module matters

The origins of the Cold War explain how the wartime allies became enemies and how Europe split into two armed camps that defined the second half of the twentieth century. For O-Level Elective History this is core examined content: the origins and development of the Cold War in Europe is one of the issues that can be set as the source-based case study, so you must know it in real depth. The skill the module trains is judging responsibility, deciding how far ideology, Soviet actions or American actions caused the conflict.

This guide ties together the module's dot points, each with worked detail and practice. See the subject hub at /sg-o-level/history and the full syllabus list at /sg-o-level/history/syllabus.

From alliance to rivalry

The first two dot points trace how cooperation gave way to confrontation.

The breakdown of the wartime alliance. The ideological clash between capitalism and communism, the unresolved disputes from the wartime conferences, and the mutual suspicion that turned the Grand Alliance into a Cold War. Study the breakdown of the wartime alliance.
The Truman Doctrine and Marshall Plan. The policy of containment, American economic aid to rebuild Europe, the Soviet reaction and why these deepened the division. See the Truman Doctrine and Marshall Plan.

Crisis and the division of Europe

The next two dot points cover the first great crisis and the alliances it produced.

The Berlin Blockade and airlift. The division of Germany and Berlin, why Stalin blockaded West Berlin in 1948, the Western airlift, and how the crisis confirmed the division. Work through the Berlin Blockade and airlift.
The formation of NATO and the Warsaw Pact. Why the West formed NATO in 1949, why the Soviet bloc formed the Warsaw Pact in 1955, and how the rival alliances completed the division of Europe. See the formation of NATO and the Warsaw Pact.

Judging who was responsible

The classic essay asks "how far" the Cold War was caused by Soviet actions, or by American actions, or by ideology. The strongest answers show that responsibility was shared and the causes interacted: deep ideological differences made conflict likely, while each side's actions, the Soviet grip on Eastern Europe and the American policy of containment, were also read by the other as aggression. Weigh the named factor against the others and decide.

Using sources in a source-based case study on the Cold War's origins

The source-based case study (Section A) may set sources on the early Cold War. Here is how to handle a typical hypothesis, such as "The Soviet Union was to blame for the start of the Cold War."

step 1: Comprehend and infer

Read each source for what it says and what it implies. A Western cartoon may portray Stalin as an aggressor; a Soviet source may present Western actions as a threat. Note the inference, not just the surface content.

step 2: Consider provenance and purpose

Check who produced each source, when and why. A Soviet government poster has a clear purpose of justifying Soviet policy and blaming the West, which affects its reliability and shapes the message it intends for its audience.

step 3: Cross-reference the sources

Compare the sources for agreement and disagreement. Where two sources from different sides clash on who is to blame, explain the clash through their provenance and viewpoint, which is exactly the comparison skill the case study rewards.

step 4: Reach a judgement on the hypothesis

Use the sources together with your own knowledge to evaluate the hypothesis. A strong answer might conclude that the sources show blame was contested and that, while Soviet actions in Eastern Europe alarmed the West, American responses alarmed the Soviets, so responsibility was shared. The mark comes from the supported judgement, not from paraphrasing each source.

Check your knowledge

Try these under timed conditions, then test yourself with the module quiz.

Explain two reasons why the wartime alliance broke down after 1945. (4 marks)
State what is meant by the policy of containment. (2 marks)
Explain how the Marshall Plan strengthened Western Europe against communism. (3 marks)
State why Stalin blockaded West Berlin in 1948. (2 marks)
Explain how NATO and the Warsaw Pact completed the division of Europe. (3 marks)

Sources & how we know this

Singapore-Cambridge GCE O-Level Humanities (Elective History) Syllabus 2174 — Singapore Examinations and Assessment Board (2026)