1) UK holds Russia responsible for chemical attack 2) Tillerson says Russia did it 3) Trump won't say Russia did it 4) Trump fires Tillerson At best, Trump's too incompetent to realize it would look like he fired Tillerson over Russia accusation At worst, well... it's a lot worse
There's no implication of invoking Article 5. It's a serious threshold--UK and US leaders have issued many accusations against USSR/Russia without calling for Article 5. No single member can invoke. They have to ask NATO to do it. Otherwise, yes, that's the nightmare scenario.