British foreign policy has varied over the centuries and cannot be traced to any one single government's policies. The United Kingdom of Great Britain, unifying the crowns of Scotland and England, resolved centuries of rivalry between the two kingdoms that had been exploited by neighboring rivals, primarily France. After this time, the kingdom's foreign policy focused on overseas colonies. After losing the American Revolutionary War, the British developed their dominance in India. In 1801, acts of parliament unified the United Kingdom of Great Britain with that of Ireland, resolving further local disputes. The victory over Napoleon established the United Kingdom as the preeminent power in Europe, leading to a period called by some the Pax Brittanica. The British joined other European states in the rush to dominate as much of Asia and Africa as possible, eventually giving the unified kingdom significant control over Africa and broad control of southern Asia. By the end of the 19th century, the United Kingdom was considered the preeminent power in the world, its legacy left in the widespread use of English worldwide.
With the rise of European nationalism, the eruption of World War I exposed the United Kingdom to mass warfare on its home shores, beginning an extended period of close alliance with the United States. Despite the allied victory with France and the United States, post-war policies led to the more destructive and financially crippling Second World War, where the British would defend lines around the world and expend much effort to command the loyalty of colonized subjects in defeating the Axis Powers. Following the war, separatist movements in British colonies the world over gained strength as the British had difficulty sustaining the overstretched empire. Resolving to consolidate the country's reach, the United Kingdom withdrew from areas in the Middle East and southern Asia, leading to the independence of several new countries and a flowering of conflicts between rival groups in the British withdrawals' wake.
Despite this, British influence has remained strong worldwide. The British military is substantially strong and in a perpetual alliance with the United States. The United Kingdom, as one of the five official winners of World War II, maintains its own seat on the United Nations Security Council. Its enormous economy also makes it a prominent player on the global economic stage. Worker shortages and unrest in former colonial holdings led to a massive wave of immigrants from the former British subjects. The British have also joined several international organizations and alliances, mainly the European Economic Community, which became the European Union in 1992. The British also maintain a leading role in the North Atlantic Treaty Organization, a military alliance primarily led by the United States.
British foreign policy has been closely aligned with American foreign policy, occasionally causing tremendous domestic discontent in the United Kingdom. Since World War I, the two countries have maintained close military ties, which have been boosted by a common language and many would say a common culture. The two countries share military research, development and mutually buy and sell technology. The British armed forces have been the key ally of the United States in the wars in Afghanistan and Iraq. However, British plans to precede American withdrawals from both countries have caused ruptures in the relationship with both the White House of George W. Bush and administration of Barack Obama. Regardless of differences, the respective president and prime minister of both countries have developed close personal relationships in recent years, even when either office is vacated and a new individual assumes the seat.
The relationship between the United States and United Kingdom is considered the most important and to constitute a cornserstone of British foreign policy. It has been termed the "Special Relationship" following the words of Prime Minister Winston Churchill in 1946. It has been used repeatedly, especially by former British Prime Minister Tony Blair. The extent of that cooperation is seen to be based on a plethora of cultural similarities and common historical origins for the peoples between the two countries. The World Wars helped to solidify a military alliance that persisted through the Cold War. Going back to World War I, divisions in American society kept the country from pursuing a non-neutral policy. Political expedience led members of the government to support alignment with the United Kingdom, but established Americans also supported the British over the German-led alliance during the war.
Despite cool periods, the personal amicability between President Ronald Reagan and Prime Minister Margaret Thatcher indicated a reinvigoration of the relationship. The two states supported each other's controversial wars during that period, and the two have functioned more or less as a unitary force during the Afghan and Iraqi wars.