Ukraine

The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.

It technically only says all parties will take what actions they deem necessary after an attack. Which could just be sending weapons similar to what they've been doing. The power of NATO comes from the military integration and economic mixing such that the incentives are aligned for one country to defend the others, not just the words on the page of the treaty.