The only economies capable of waging a war "from the other side" on NATO are China and Russia, and the keys of their economies care too much about not dying to let some lunatic take over these states to wage a world war.
The early 20th century was a transitionary period where plutocrats learned that war between powers was no longer profitable. As soon as it stopped being an economically beneficial act to try to conquer, people stopped doing it. That is why Russia is able to still conquer foreign land - they know nobody cares enough to stop them, so they can just grab it for free. They also know they could never try that against a NATO member or first world nation, because it would end them. They are too rational in their behavior for that kind of stupidity.
Russia could keep taking non-NATO land, while new nations seek admission to NATO. At some point, Russia could dispute the status of a new-NATO member claiming they already held that territory or some such.
You can't know what increased tensions will lead to. Bay of Pigs and the cold war were very real. Tensions in the South China Sea and North Korea are real. The fondness that the US president holds for murderous conservative leaders, and his disdain for left-leaning leaders, are real.
I would mention the pact the UK signed with Poland days before Germany decided to invade anyway, however, you claim the circumstances are different now and that history is irrelevant. I'd say ignoring history is foolish.
If only one could guarantee such a thing. Yet we know there are madmen out there, angry enough to push that button, or lead armies to conquer more territory. Humans aren't much different today than we were 1,000 years ago.