They could invoke but take no real action, depending on the state of the rest of the world at the time he made his move. I still think we sent a subtle message to only put troops in Poland, that we wouldn’t necessarily fight as hard for Lithuania.
The Poles, however, may react in way to force us into it anyway, especially since its in their interest to do so.
The US has been rotating an armored battalion into Lithuania, non stop, for multiple years.
They also engage with Polish troops and train with them inside Lithuania.
There have also been Brigade sized training with 13 NATO countries, fairly recently...in Exercise Iron Wolf.