How far back in history does an event have to happen before something becomes "fixed" in the past?

As an example, Israel is locked into the idea that the West Bank and Gaza must be returned to Palestine, but the conflicts leading to that happened 40 and 60 years ago. In contrast, the US defeated Mexico in 1847 and took Texas, California and the entire Southwestern US. If Israel is expected to return lands captured in war, shouldn’t other countries be expected to do the same? Does the fact that the Mexican war happened more than 150 years ago change things? If so, what is the time cutoff and why?