There was a 1919 agreement between Switzerland and Liechtenstein that Swiss diplomats would represent impoverished Liechtenstein in world affairs. In 1945 the Swiss rejected the idea of joining the newly established UN on the grounds that it would affect the status of their neutrality. Liechtenstein went along with this, but decided in 1989 that, with the ending of the Cold War, this was no longer a reason not to join the UN. It would allow them to participate in working for world peace, and, (since they'd become a very wealthy country by now), they were keen to help out in the various UN aid foundations. So they applied and were accepted as a UN member in 1990. The Swiss, by the way, didn't join the UN until after a very close referendum result in 2002.