Please go read Article 5. It does not require a military response. I don't know how so many have been misled to believe otherwise. The text is right there.
"The Parties agree that [...] if such an armed attack occurs, each of them [...] will assist [...] by taking [...] such action as it deems necessary [...]"
Each party gets to act however it deems necessary. Which could be, you know, "tweet angrily." Or "do literally nothing."