Both the first and second world wars were started by the UK because we went to the aid of a country with which we had treaty obligations. Germany invaded Belgium and then Poland. When Iraq invaded Kuwait the same principle applied. After that things got complicated.
With the break up of Yugoslavia after Tito died the Serbs committed atrocities against the Bosnians despite a UN peacekeeping force. Tony Blair persuaded Bill Clinton to help him defend the Bosnians, even though their action was very late and subsequently the Kosovans were also protected from the Serbs. Blair had been encouraged in all this by the success of the invasion of Sierra Leone to protect the government against Charles Taylor in a rebellion funded by blood diamonds.
I see today that Tony Blair is advocating regime change in Iran and Syria. On television yesterday there was a worry that Iraq is now an Iranian vassal state. Yer no-one has advocated invading Zimbabwe or Byelorussia where there are really nasty dictatorships.
Interference in the internal affairs of another country is forbidden by the Treaty of Westphalia. Taking sides in a civil war is dangerous if you are found to have supported the loser (as Tony Blair did in Libya).
What do you do when pitted against a private army. In Sierra Leone it worked out well, but it took 10 years for the might of the US to kill Osama bin Laden. The private army is unlikely to obey the Geneva conventions. Should you? War has become much more difficult.