After the end of WWII, Germany was divided into four parts, each part to be controlled by one of the Allied countries: the U.S., Britain, France, and the Soviet Union. As tension between the democratic countries and the communist USSR grew during the cold war, Germany was divided into just two parts: the Federal Republic of Germany, with West Berlin, and the German Democratic Republic, with East Berlin.
The west was democratic and capitalist. The east became an authoritarian government with a Soviet-styled economy. The West Germans economy flourished, making many East Germans wanting to move into those areas. To prevent this, the government established a clear border in Berlin, and stopped East Germans from moving into the west.
Stalin met with East German leaders and expressed his displeasure that westerners could still move freely in the east. Stalin advised the East Germans as follows: "The demarcation line between East and West Germany should be considered a border – and not just any border, but a dangerous one ... The Germans will guard the line of defense with their lives."
The reason Stalin was so firm on this was a logical one. He was trying to protect his power. The more access citizens of authoritarian countries had with those of free countries, the more the feeling of unrest could grow, which would topple the dictators. This did eventually happen - just not for another 20 + years.
The Soviet Union built the Berlin Wall separating communist East Germany from democratic West Germany because the fence they erected was not doing its job. Yes, it started as a fence and over time became a wall. The Soviet Union's offical reason was that it wanted its borders identified, and to guard against intruders. The great irony of their offical reason was that intruders trying to get in was not the problem, the problem was trying to keep people on "their" side of the wall.
The Marshall Plan, which was enforced by USA, to stop communism from spreading, affected the UDA's side of Berlin, West Berlin hugely. West Berlin was in a greater, more economically powerful situation, than East Berlin, which was ruled by the communists, the Soviet Union. As a result, Germans from East Berlin began to move to West for a greater quality of life, this was an embarrassment for the Soviet Union. Khruschev, the leader of the Soviet Union, decided to build a permanent brick wall, lined with soldiers, to force the Eastern citizens from fleeing to the West, this wall named - The Berlin Wall.
At the aftermath of the 2nd World War, Germany was split into four parts, each given to Amerca, France, Britian and Russia, the Allied countries who took part in the war. There was repeated conflicts between the democratic leaders and the communist USSR, so to stop heightened tension resulting in a full-scale war, Germany was split into two main parts, the Western side the German Federal Republic and the Eastern Germany the German Democratic Republic by the USSR in 1949.
The Western side was a democratic and capitalist continent, comprising of the USA and other great powers while the eastern side was a communist continent, comprising of USSR. The USA and other great powers decided to create a new currenct for Germany, helping the Western side to recover from the carnage suffered from the effects of the world war, and also held democratic elections there. Fearing of an overlapping strength of the Western Germany, Stalin perceived this expanding power a threat to his country's survival, so he planned to seize control of West Berlin by ordering all his soldiers to block all roads, rails and canals linking between the eastern and western sides of Gemany in June 1948. He did that to ensure none of the eastern citizens would take this golden opportunity to defect to the democratic countries, the West and to stay loyal to the East.
This Berlin blockade ended at MAY 1949, signifying that the West had worn the major battle against USSR.