Canada–United States relations covers more than two centuries, marked by a shared British colonial heritage, conflict during the early years of the U.S., and the eventual development of one of the most successful international relationships in the modern world. The most serious breach in the relationship was the War of 1812, which saw both sides try to invade the other, and both failed, leaving the status quo. Friendship was solidified in the 20th century with the shared experience of the world wars and a close alliance during the Cold War. Canada and the United States are currently the world's largest trading partners, share the world's longest border, and have significant interoperability within the defense sphere. Modern difficulties have included repeated trade disputes (despite a continental trade agreement), environmental concerns, and debates over immigration and the movement of people across the shared border. While the foreign policies of the neighbors have been largely aligned for much of the post-war era, significant disputes have arisen, including over the Vietnam War, the status of Cuba, the Iraq War and the War on Terror. At this time, the Canadian dollar has gained considerably versus the U.S. dollar with as yet unknown consequences.