Guess that would be a good reason for the rest of the world to get involved, right?
Yes, I think the international community should get involved when a nation invades another without justification, like the US invasion of Iraq, for instance. However, NATO is not “the rest of the world.” NATO consists of 32 nations (out of 195), all of which are located in Europe and North America, and more than 2/3 of its funding comes from just one country: the United States.