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The big idea: The USA and USSR built enough nuclear weapons to destroy the world many times over.
Strangely, this deadly balance helped keep the peace — neither side dared strike first.
In 1945 the USA was the only country with the atomic bomb. This gave it a huge advantage, and the Soviet Union felt threatened and exposed.
But that lead did not last long. In 1949 the USSR tested its own atomic bomb, shocking America and starting a race that would last for decades.
- US atomic monopoly (1945–1949) — for four years only America had the bomb, which it had used on Hiroshima and Nagasaki in 1945.
- Soviet A-bomb (1949) — the USSR caught up far sooner than the West expected, ending the American monopoly.
- The hydrogen bomb — the USA tested the far more powerful H-bomb in 1952; the USSR followed in 1953, raising the stakes hugely.
- The delivery race — both sides raced to build long-range bombers and then missiles to carry these warheads across the world.
The hydrogen bomb was a terrifying leap. A single one could wipe out an entire city in an instant.
Mutually Assured Destruction (MAD): By the 1960s both sides could destroy each other even after being hit first.
This grim balance was called Mutually Assured Destruction, or MAD. Because attacking meant your own certain death, neither superpower ever pressed the button.
MAD is a strange idea, but it may have kept the Cold War 'cold'. The fear of total destruction made both sides very careful, especially after the Cuban Missile Crisis of 1962 scared the world.
Why MAD mattered: MAD explains why the two superpowers competed everywhere else — spies, sport, space, proxy wars — but never fought each other directly. A direct war would have destroyed them both.
By the early 1970s both superpowers were exhausted and worried. Nuclear weapons were hugely expensive, and the fear of destruction was constant.
So they tried something new — talking instead of threatening. This easing of tension was called détente.
SALT I (1972)
The Strategic Arms Limitation Talks led to the first big agreement to limit the number of nuclear missiles each side could keep. It did not cut weapons, but it stopped the race growing endlessly.
The ABM Treaty (1972)
Signed alongside SALT I, this limited each side's anti-missile defences. The clever logic was that if neither could defend itself, neither would risk a first strike — MAD stayed in place.
The Helsinki Accords (1975)
35 nations agreed to accept Europe's existing borders and to respect human rights. The West accepted Soviet control of Eastern Europe; in return, the USSR promised freedoms it would later be pressured to honour.
Better dialogue
Leaders now met at summits, signed deals, and even cooperated in space. The tone of the Cold War softened for most of the 1970s.
Détente = talk, treaties, and trust: SALT, ABM, Helsinki.
The Helsinki human-rights trap: The USSR signed Helsinki mainly to win acceptance of its borders. But by promising to respect human rights, it gave its own critics — like Soviet dissidents — a powerful weapon to hold the government to its word.
Détente did not last. At the end of the 1970s tension flared again, in what historians call the Second Cold War.
Détente (1970s) — tension easing
- SALT I limits nuclear missiles (1972)
- ABM Treaty preserves the MAD balance (1972)
- Helsinki accepts Europe's borders (1975)
- Regular summits and warmer dialogue
Second Cold War (early 1980s) — tension returning
- Soviet invasion of Afghanistan (1979)
- USA boycotts the 1980 Moscow Olympics
- Reagan's huge arms build-up
- Reagan calls the USSR an 'Evil Empire' (1983)
The turning point: Afghanistan, 1979: In December 1979 the USSR invaded Afghanistan to prop up a friendly communist government. The West was furious, saw it as aggressive expansion, and détente collapsed almost overnight.
The new US President, Ronald Reagan, took a hard line from 1981. He massively increased military spending and used tough language, famously calling the Soviet Union an 'Evil Empire' in 1983.
Why the mood changed: Afghanistan destroyed Western trust, and Reagan's build-up piled pressure on the Soviet economy. This costly new rivalry set the stage for a leader who wanted to end it — Mikhail Gorbachev.
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In 1985 a new, younger leader took charge of the USSR — Mikhail Gorbachev. He inherited a country that was falling behind and could no longer afford the arms race.
Gorbachev believed the only way to save the Soviet Union was to reform it. His ideas would end up ending the Cold War altogether.
- Glasnost — glasnost allowed people to criticise the government for the first time in decades.
- Perestroika — perestroika tried to fix the failing, over-controlled Soviet economy.
- 'New Thinking' — Gorbachev's new approach to foreign policy: cooperate with the West and stop treating it as an enemy.
- Less control abroad — he told Eastern European countries the USSR would no longer send troops to keep communist governments in power.
The INF Treaty (1987): Gorbachev and Reagan built a real friendship. In 1987 they signed the INF Treaty, the first deal to actually destroy a whole category of nuclear weapons rather than just limit them. It was a huge breakthrough.
Once people knew Soviet tanks would not roll in, the peoples of Eastern Europe rose up. The year 1989 saw one communist government fall after another, mostly peacefully.
The revolutions of 1989
Poland, Hungary, Czechoslovakia and others threw off communist rule during 1989. Because Gorbachev refused to intervene, the old Soviet grip on Eastern Europe simply melted away.
The Berlin Wall falls (Nov 1989)
The Wall had split Berlin since 1961 and was the great symbol of the divided world. On 9 November 1989 crowds broke through and began tearing it down — the Cold War's most famous image.
German reunification (1990)
With the Wall gone, East and West Germany joined back together as one country in October 1990. The division of Germany, where the Cold War began, was over.
The USSR collapses (1991)
Reforms unleashed forces Gorbachev could not control. In December 1991 the Soviet Union broke apart into separate independent countries, and the Cold War was finally over.
1989 revolutions → Wall falls → Germany reunites (1990) → USSR collapses (1991).
A key debate: Historians argue about who ended the Cold War. Some credit Reagan's tough pressure, which bankrupted the USSR. Others credit Gorbachev's reforms and 'New Thinking'. Most agree it took both — plus the Soviet economy's own deep weakness.
Gorbachev won the Nobel Peace Prize in 1990 for helping end the Cold War peacefully. Yet at home he is often blamed for the collapse of his own country.