"The modern U.S.-UK relationship was forged during the Second World War. It was cemented during the Cold War, as both countries worked together bilaterally and within the North Atlantic Treaty Organization (NATO) to counter the threat of the Soviet Union. The United States and the UK are two of the five permanent members of the United Nations Security Council, and both are founding members of NATO. In the early 1990s, the UK was an important U.S. ally in the first Gulf War, and the two countries later worked together in stabilization and peacekeeping operations in the Balkans. The UK was the leading U.S. ally in the 2003 invasion of Iraq and subsequent stabilization operations, remains the largest non-U.S. contributor to the NATO-led mission in Afghanistan, and took a leading role in alliance operations in Libya in 2011. It is also an important U.S. partner in efforts to pressure Iran over its nuclear activities, and to combat international terrorism. The UK is the seventh-largest economy in the world and a major financial center. The United States and the UK share an extensive and mutually beneficial trade and economic relationship, and each is the other's largest foreign investor."
CRS Report for Congress, RL33105