NATO Article 5 was specifically worded by the USA to avoid any commitment to automatic armed response. During the Cold War, the UK — at great expense (comparable to $US250B per year today) — was the key hawk. After The Wall fell, the UK (and the US) reduced capacity towards peacetime levels and pivoted away from Europe. British MPs and the public are increasingly sceptical about foreign wars.
For a number of years, tensions between Britain and the European Union institutions have grow. The EU institutional response to the recent failed coup in Turkey increases tensions. Since 2012, the EU institutions enjoy a special relationship with NATO. Doubt exists about whether Turkey and British MPs would quickly agree to a war with Russia managed by EU institutions. Military capacity and preparedness in the EU is otherwise low. In short, today NATO may talk like a Hawk but walks like a Dove.
The US has a large and extremely capable military but is overstretched. The US would be unable to pivot significant resources into Europe before the Baltic states are overrun. Without a capable and credible European response, the game is very different for the US — whether to launch a World War against a nuclear armed power to avenge allies already lost.
But NATO was never a nuclear shield, and promising to end the world is not a credible deterrent to asymmetric or information warfare.