The UN Is not going to invade/occupy/invoke martial law in the United States. Want proof? See how they're doing in Bosnia, Rwanda, Chad, Sudan, the Congo, etc.
THERE ARE NO UN TROOPS!!! The member states of the UN sometimes rent some of their military to the UN for missions, but they're dismally bad at even "peacekeeping" let alone enforcing martial law upon the people of the United States.
In the Balkans in the mid 1990's a couple of UN soldiers actually starved to death because the UN couldn't even feed them. The troops from Bangladesh were deployed to the Balkans without bolts for their rifles and without ammunition.
It ain't going to happen. No "blue helmets, no black helicopters, no UN Air Force, NOPE!
Who says they’re UN trucks? And if they ARE UN trucks, who says ONLY THE UN can occupy them when martial law goes into effect?