Morse Code during the opening title says: One got away
Casus Belli in Latin literally means occasion of war.
The episode title is "Casus Belli", which is Latin for "occasion for war". As a military and political term it is used to describe an act or event that provokes or justifies war. Specifically casus belli involves direct attacks or threats against the nation declaring war; whereas casus foederis (Latin for cause for alliance) is the term for attacks or threats against the declaring nation's ally, typically when two nations have a mutual defense pact an act of war on one nation is considered to be an act of war on both nations. Best example is NATO (North Atlantic Treaty Organization), which is a mutual defense pact signed by over 30 nations that border the Atlantic Ocean, members include the US, Canada, Great Brittan, France, Italy and many other European nations. If any member of NATO is attacked the other countries in NATO are obligated to help defend that nation if asked to do so.