That is complicated. And historically simply not true. Being subject/citizen of one country means that in case of war, your property is legitimate to be anything from damaged to seized by opposing parties in the war. The fact that we hadn't had any kind of real war in since WW2 means that no-one has that experience in recent history.

I'm aware that there have been significant amounts of armed conflict in the mean time, but the last conflict between states that ran similar to historic wars was the Korean war (which technically hasn't ended). Most wars the US was involved in where more similar to colonial expeditions (seizing local resources, defending local bases)