The United States was involved in the reunification of Germany in 1990 because it was the most powerful nation in the world at that time and because it was one of the four (along with Britain, France, and the Soviet Union) victorious Allied powers from WWII. The US mainly helped German reunification along by pushing the other major powers to accept it.
Because of the legacies of the two world wars, there were many European countries that were at least somewhat worried about the prospect of German reunification. France and the Soviet Union had both been invaded by Germany during the wars. Britain had not been invaded but had been threatened with invasion and had been attacked by air in WWII. For these reasons, all three countries were at least somewhat opposed to reunification. The countries had some power to impede reunification because they all shared power over Berlin because of the deals made at the end of WWII.
The main role of the US in the reunification was to push the other countries to allow German reunification. The US, being across the Atlantic, did not have the same fear of reunification. It felt that reunification would be a good thing. Therefore, the US exerted its diplomatic power to help convince the countries of Europe to allow Germany to reunite. This was the main thing that the US “had to do” with German reunification.
During the Cold War, Germany was divided into 4 parts each occupied by a different country. These four countries were France, Britain, the United States, and the Soviet Union (Russia). The parts of Germany controlled by France, Britain and the United States formed the Federal Republic of Germany, commonly known as West Germany (democratic), while the Soviets formed a German Democratic Republic, East Germany (communist).
East Germany was at first very difficult to leave. However, after the Soviet leader, Gorbachev started opening up to the west, the west soon started protesting to open up themselves to the east. As a result, the west and east fully opened their border that split Germany apart. East Germany merged with the West to form a democracy, and signed a treaty called the Treaty on the Final Settlement with Respect to Germany, commonly known as the Two+Four Treaty with the Soviets, France, Britain, and the United States, proposing the unification of the country Germany.