https://www.un.org/securitycouncil/content/purposes-and-prin... : "prohibits the threat or use of force and calls on all Members to respect the sovereignty, territorial integrity and political independence of other States".
The post-WW2 peace structure was built around a prohibition on aggressive war and sub-war violations of the integrity of other states. This hasn't always held, and once we get into a situation where there are state-power-but-not-a-state actors involved (who are by definition not part of the UN!) things get messy.
I do think the US breach of this principle by invading Iraq did a colossal amount of damage to this principle, both morally and practically in that the collapse of the (repressive) Iraqi state caused the rise of non-state groups which destabilized Syria. The collapse of Yemen has been going on for a while with very little caring from outside, and now it's a venue for the proxy war between Iran and everyone else in the area (mostly Israel, but also Saudi).
It's also an easy mistake to blame everything on the US and Israel; there are a lot of different groups looking to seize various sorts of power in various places at gunpoint, with varying outside backers with varying levels of control.
There was even at one point indirect cooperation between the US and Iran to defeat the only group that was an even worse influence on the stability of the region than either of them: ISIS. https://www.theguardian.com/world/2014/dec/03/us-iran-forced...