NATO is a great example. The US funded about 22% of the NATO budget last year, which seems about consistent with its size relative to the other members: http://www.nato.int/nato_static_fl2014/assets/pictures/stock...
NATO members absolutely should spend what they agreed to spend, but we aren't forced to spend 3.6% of our budget on defense because they don't. That's something we choose to do, and we could stop if we wanted to.