Take the Iraq War, for instance. Russia and China went against the U.S., the pavlovian nature of which makes their votes irrelevant. France and Germany were a different story. It wasn't until after the invasion that we learned why they went against the rest of the world. These two countries were involved in the largest example of fraud in human history (by the numbers), which certainly provided the motive to stay out of Iraq. Every other country in the world that matters, save those four, went along with the action -- 39 in all, including the UK, Spain, Italy, Japan, and Australia.
The UN is a tool for us (and others) to use, but it is not the only tool.