In the summer of 2005 America and Europe wanted Israel's disengagement from Gaza to be complete. It was really important to them. So by way of convincing Israel there'd be no danger in opening the border between Gaza and Egypt, everyone signed a treaty. The Americans signed, the EU signed, Egypt signed, Israel signed, and the PA signed. The treaty described how European professionals would watch the movement of goods and people in and out of Gaza, and they'd be nice to the Palestinians but also protect Israel's interests.
Then, about six months later, Hamas won the Palestinian elections and it all started unraveling. By the time Hamas violently threw the PA security forces out of Gaza, nothing was left of the treaty. This week, less than six years later, a new Egyptian regime (a temporary one, perhaps) opened its border with Gaza, and no-one even pretends to respect the defunct treaty, nor to care.
Meanwhile, far away in Africa, Sudan is using military force to change its border with the emerging country of South Sudan. If you look hard enough you can find mention of this at well-informed websites such as the Economist; the Guardian has even mentioned it repeatedly (here, here, here and here). So far as I see, words such as "illegal according to international law" or their synonyms don't make it into any of these reports; though here's a blogger who's pretty explicit about what's going on. A blogger. At one point the Guardian does use the term "disputed territory", however, thereby demonstrating that there's at least one keyboard in their system that has the keys necessary for such words.
Just another example among many that international law may be useful when peaceful democratic states suchas Iceland and Britain need to resolve disagreements about fishing rights in the Northern Atlantic, but it's useless when coping with international conflicts.
My point, of course, is not to bemoan the irrelevance of international law; on the contrary, my position is that international law needn't be brought into armed conflicts in the first place, not as a tool for resolving them. The mere fact that they're violent proves that at least one side is willing to kill so as to promote their interests, so why would they be frightened off by some words in a dusty book on a shelf. National interests are what makes international relations happen, not laws.