Having an AAAD (A2/AD) defense system poses a threat to bombing planes, missiles etc.. Defensive systems raise the cost of an invasion or bombing runs. You can

Having an AAAD (A2/AD) defense system poses a threat to bombing planes, missiles etc.. Defensive systems raise the cost of an invasion or bombing runs. You can threaten countries easily or in an easier manner when we deny them systems like the S400. Would we have bombed Vietnam, Laos and everywhere else so indiscriminately if the had proper defensive systems in place. https://en.wikipedia.org/wiki/Area_denial_weapon https://en.wikipedia.org/wiki/Area_denial_weapon