In many of our movies, in the beginning, the hero is shown as a very weak individual. The villain constantly beats him up. But in the course of the movie, the situation slowly begins to change; and by the end, the roles are reversed. It's the hero who is in a dominant position and the villain in a weak one. When you say that the boot is on the other foot, you mean that the situation is the opposite of what it was earlier. In other words, the tables have turned. Americans tend to say, `the shoe is on the other foot'.
The Americans are realizing that they can't get other countries to do what they want them to. The boot is on the other foot now.