Would the violation of a diplomatic bag be a casus belli for US?
I've just seen the last episode of "Homeland" and I have this question.
Precisely.Only if the two countries are so mistrustful and already on the brink of war that opening a bag would be the "straw that breaks the camel". Even if Chinese agents open a US bag (or vice versa) it will likely be quickly swept aside and someone in the intelligence department is scapegoated.