The Cold war is a period of sustained political and military duress and unrest between the Western Superpowers, viz. The USA Of America , and its NATO allies , and theÂ communist world, led by the erstwhileÂ Soviet Union, itsÂ satellite states and allies 1947 - 1991. It occurred post the 2nd world war, and was a "mixture of religious crusade in favour of one ideology or the other, and of the most ruthless power politics, striking out for advantage or expansion not only in Europe but all over the world." 
"The war was "cold" only in that the USA and USSR never fought each other in a direct military confrontation, but both superpowers threatened each o withÂ nuclear annihilationÂ and participated frequently in "proxy wars" by supporting allied nations in numerous "hot" wars in places likeÂ Korean,Â Vietnam), andÂ Angola. The Cold War defined both countries' foreign policies through the second half of the twentieth century, as Americans and Soviets competed for allies to maintain and widen their respective spheres of influence around the world. Each side viewed the Cold War as a battle between civilizations; in the worldwide clash between American capitalism and Soviet Communism, only one could prevail. For more than forty years, the Soviet-American conflict hung heavy over global affairs, shaping the world with massiveÂ military buildups, a never-endingÂ nuclear arms race, intensiveÂ espionage, and fierceÂ technological competitionÂ as each side tried to gain the upper hand in preparation for the thermonuclear "hot war"Â all humans feared would someday come." 
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The causes of the Cold War.
We can divide the causes of the Cold War into two types of causes viz. The underlying causes and the immediate causes.
The Underlying Causes are :-
(i) Ideological Causes:
The USA and the Soviet Union represent two opposing systems of government. In the USA, the government is elected by free elections i.e. the people can form political parties to voice their political opinions. They also possess the right of assembly, of speech and of the press. In the Soviet Union, the government is formed by the Communist Party. The people do not have the right to form their own political parties. They do not enjoy the right of assembly, of speech and of the press. Since these two systems of government are diametrically opposed to one another, there can be little compromise between the USA and the Soviet Union.
(ii) Economic Causes:
The USA wanted to encourage free trade throughout the world and the USSR wanted to shield off its own sphere from international commerce. Russia feared that trade with the West would involve the risk of Russia being opened to western influences which would have eroded the strength of the totalitarian regime. These differences led to much ill feeling between the USA and the USSR.
Immediate Causes of Cold war:
Incipient conflict between the Soviet Union and the United States began at the peace-time conferences. Their conflict was intensified after President Truman declared the Truman Doctrine and launched the Marshall Plan in 1947.
(i) Extension of Russian influence in Europe:
Even before the end of the Second World War, the Soviet Union had gradually extended its influence in Europe. By 1944, it controlled a large part of Eastern Europe. By 1945, at the Yalta Conference, the Soviet Union obtained the Curzon Line as its new boundary line with Poland and also gained control of the eastern part of Germany.
As the war was drawing to a close in May 1945, the Soviet Union quickly consolidated its control of eastern Europe.
(ii) The reactions of the United States:
Despite the increasing Russian influence in eastern and central Europe, many politicians in the United States were optimistic about the chances of co-operation with the Soviet Union after the war and did not advocate strong resistance against Russian expansion.
But from May 1945 onwards, the situation was changed. The U.S. government favoured a policy of strong resistance against Russia.
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This was because President Roosevelt -who was optimistic, believed that though eastern Europe had fallen under the influence of Russia, Russia would keep its promise made at Yalta by setting up freely elected parliamentary governments in the area and so did not advocate strong resistance against Russian expansion- died on April 12, 1945. He was succeeded by Harry S. Truman who was a complete contrast to Roosevelt. He did not believe the communists. He thought that the communists would not set up democratic governments in Eastern Europe. He also believed that after the Soviet Union had established its control in Eastern Europe, it would continue to extend its influence into Western Europe. Thus President Truman favoured a policy of strong resistance against Russian expansion.
TheÂ second reasonÂ was that just before the Potsdam Conference was to take place, the United States had successfully exploded its atomic bomb. President Truman thought that since the United States alone possessed the atomic bomb, it could adopt a stiff attitude towards Russian expansion in Europe.
TheÂ third reasonÂ was that President Truman was disgusted at the non-co-operative attitude of the Russians at the Potsdam Conference where Russia was determined to exact heavy reparations from Germany. Russia also accused the British of upholding a reactionary monarchy in Greece and supporting an Italian Fascist regime in Trieste. Stalin also blocked Truman's proposal on the internationalization of all principal waterways.
(iii) Poor relations between the United States and the Soviet Union:
The deteriorating relations between the Soviet UnionÂ and the United States were reflected in two minor incidents in the year. Land-Lease was abruptly terminated by the United States and the Russian request for American economic aid for the purposes of post-war reconstruction was ignored by the government of the United States.Â (During the Second World War, the U.S. supplied much war material to the Allied nations through a Lend and Lease programme. As the Lend and Lease programme was suddenly stopped, the war-ravaged Soviet Union could not obtain American material support to help its post-war economic reconstruction.)
The poor relations between the East and West were also reflected in a speech by Churchill. In March 1946, Churchill made a speech at Fulton, Missouri in which he said, "From Stettin in the Baltic to Trieste in the Adriatic, an iron curtain has descended across the continent .... Behind that line lie all the capitals of the central and eastern Europe - all are subject in one form or another to not only to Soviet influence but also to a very high and increasing control from Moscow." 
(iii) End of World War II and events leading up to the Cold War
Once the World War II ended, it was obvious that the alliance between USA, Britain, and USSR would end, as there were tension between the west and the east.
The USSR joined the allied forces only after Hitler betrayed them. Also, the huge difference in their political and economic ideologies would not let them stay allied for a long time.
The events that led to the Cold war are -
1) Yalta Conference (February 1945) - Held during the war, on the surface, the Yalta
conference seemed successful.Â Â
At Yalta, the negotiations went very much in Stalin's favour, but this was because
Roosevelt wanted Russian help in the Pacific, and was prepared to agree to almost
anything as long as Stalin agreed to go to war with Japan.Â
Although the Conference appeared successful, however, behind the scenes, tension was
growing, particularly about reparations, and about Poland.Â Â
Â Â After the conference, Churchill wrote to Roosevelt that 'The Soviet Union has become a
danger to the free world.'Â And on their return home both he and Roosevelt were
criticized for giving away too much to the Soviets:
The Potsdam Conference (July 1945) - Serious differences arose over the future of Germany and Eastern Europe , and also Truman Revealed to the USSR that it had a powerful new weapon ( the atomic bomb) which kick started the Nuclear Weapons Race that characterized the Cold War.
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The Truman Doctrine (March 1947) - It was the unofficial policy of the US to stop the spread of communism in Europe and other places which conflicted with the USSR's desire to convert other countries to communism. This also led to the Marshall Plan.
The Marshall Plan (June 1947) - This was about US aid to European countries, with which they ended up forming the NATO alliance.
Summing up, we can say that the main causes of Cold War are the events that run up to it , such as the Truman Doctrine , The Marshall plan , the Potsdam conference and the Yalta Conference , and the conflicts of the USSR's and USA's political and economic ideologies.