They are always defensive in nature.
The idea that the UN hasn't kept peace is pretty laughable. I'm not sure what you expect it to do - membership is voluntary, and it is 100% better to have a belligerent, antagonist, bad faith enemy sitting at a table with you than not.
As a citizen and resident of a small nation living beside an antagonist, militant, imperial nation on the security council, the UN is not there to make sure they don't do anything bad. It's there so that when they do, they and their citizens hear the consequences.
None of which has anything to do with Peacekeepers who once deployed do not have to have "approval" to defend themselves or civilians, per the scary long document you didn't read. Do you want me to find it in a convenient YouTube format for you?