> If a terrorist crashes a truck with Portuguese plates into the US embassy in Berlin, would that mean Portugal's declared war against the USA?

At the very least, the cooperation of Portugal's authorities would be expected to determine how the truck ended up being used for the attack, and if anyone knew about how the vehicle was to be used.

I expect the same amount of cooperation from China as the flag state.

By this logic, United and American Airlines were complicit in 9/11 as well.