After the September 11 attacks, NATO invoked Article 5 of its treaty which stated that allied nations must support
a. a nation under attack.
c. a nation attacking another nation.
b. a nation in war.
d. any nation in the treaty for any reason.
The Article 5 is often quoted as "an Attack against one of us is an attack on all" - this means that the treaty requires attacking a. a nation under attack.
