The one vote of the US doesn't matter. Historically, the US has been the biggest funder of the UN, and traditionally the organisation is dependent on the dues of the US being paid. The US tends to use this as a bargaining chip, which is an awful lot more powerful than just one vote. http://en.wikipedia.org/wiki/United_States_and_the_United_Na...

Bear in mind that we're not even talking about US diplomatic efforts here, which typically sway many nations in exchange for other favours.