February 12, 2012: NATO is making its Baltic Air Policing Mission (BAPM) permanent. BAPM is nothing more than a detachment of four jet fighters from a NATO nation that moved to a Lithuanian air base and flies air patrols along the Russian border for four months. This began in 2004, when the Baltic States (Lithuania, Latvia, and Estonia) joined NATO. The Baltic States wanted some NATO presence from older members, if only to remind the Russians that something had changed. Many Russians regard the Baltic States as "lost provinces." Since the 18th century the Baltic States were part of the Russian empire. They became independent after World War I (1914-18) but were taken over again in 1940. It wasn't until 1991 that the Baltic States regained their independence and they are determined to keep things that way.
In 2008 the U.S. Air Force sent four F-15s to serve along the Russian border on BAPM duty. This sort of thing did not go down well in Russia where it still rankles that Russia was on the losing side of the Cold War. The F-15s were the second American detachment to serve in the BAPM, the first was equipped with F-16s.
It's common for NATO members to train or operate in each other's territory. After all, NATO is basically a mutual defense organization. In addition, the Baltic States do not yet have jet fighters of their own. So the BAPM provides token air defense until the Baltic States can acquire their own. That is to happen by the end of the decade.