What is Casus Belli? Translation to English and usage examples

Casus belli is a Latin phrase that translates as "a suitable occasion for declaring war" or simply "a reason for war".

The expression is used both in diplomacy and in ordinary speech. What kind of event can be a casus belli? Check out some examples:

1) Euromaidan and the expulsion of Yanukovych turned out to be a sufficient reason for the Russian leader Putin to invade Ukraine, without declaring war.

2) For Israel in 1967, the buildup of Arab armies on its borders turned out to be a casus belli. The Jews understood that the Arabs still want to fight and destroyed all enemy aircraft right on the airfields.

3) In 1939, Hitler himself fabricated a casus belli for the invasion of Poland, disguising his soldiers as Poles, who then "attacked" the German border guards.