> it's an attempt to deter aggression by expressly recognizing an attack on one member as an attack on all.

...and it appears the two mutual alliances pre-World War 1 basically ended up hastening the start of World War 1.

Why do we think that it would be different this time?

Because your example is 108 years old and no NATO nation has been attacked militarily[1] in the 73 years since the treaty was signed?

[1] With no doubt a handful of exceptions that people will point out. The Faulklands war comes to mind, where the UK deliberately did not invoke the treaty and chose to defend its territory itself, etc...

> no NATO nation has been attacked militarily in the 73 years since the treaty was signed [..]

Without a definition of "attacked militarily" I'm not sure that claim is useful. Perhaps mostly due to the concept of countries "declaring war" on each other having rather gone out of fashion?

Back in 1943, Roosevelt asked (!) the Congress declare a state of war. These days it seems it's more like "shoot first and ask [strike]questions[/strike] Congress later"

It's Falklands. And anyways, NATO as it stands today will not be attacked. But potential NATO entrants very much might be, prior to and maybe even after entering. No one believes that NATO will actually fight beyond Poland. Hell, I don't believe the Germans would fight for Poland -- Poland would fight for Poland, but that's probably it. Defensibility matters. I think Poland is defensible mainly because this time Poland is prepared and willing to defend itself long enough to embarrass the rest of NATO into helping.

That means Poland is now safe from attack.

All the new entrants being discussed are less defensible than Poland. With Ukraine being left to flap in the wind and lose, the value of discussing NATO entrance is now negative.

Wasn't there a war between two NATO countries? Turkey and Greece.

"In 1964, due to the Cyprus crisis, Greece withdrew military units from NATO forces in the Southern Mediterranean, over threats of invasion of Cyprus by fellow NATO member Turkey.[8] Later in 1974 due to the invasion of Cyprus by Turkish forces, Greece withdrew from NATO military command. Notwithstanding, the country did not withdraw entirely from the organisation, but became significantly less active.[9]

In 1980, the Greek foreign minister Konstantinos Mitsotakis made remarks about the situation where he could see Greece fully withdrawing from the organisation. However, later diplomatic pressure from the United States led to Greece fully re-integrating with the alliance.[10]"

https://en.wikipedia.org/wiki/Withdrawal_from_NATO#Greece

Deterrence relies on the to-be-deterred believing the deterrence threat. If entrant X is not defensible, it won't be defended, therefore the deterrence value of accepting entrant X is low.

No one -but no one- believes that NATO will fight a war with Russia over Finland, or over Ukraine, or Belarus, or Georgia, or at all. Certainly Putin does not. I assert certainty w/o being a mind reader and not truly knowing what's in going on in his mind, but it's pretty clear to any and all that Germans, French, British, Americans, etc, none of them want a shooting war with Russia even if it could be an easy win because a) it might not be an easy win, b) it might be ruinously costly in blood and treasure even if it were an easy win, c) it might lead to nuclear war where no one wins and everyone loses.

If, however, Finland is easily defended, and can defend itself by itself with little extra help from NATO, then that's a deterrence to invasion of Finland regardless of Finland's membership in NATO. I think the experience with the Winter and Continuation Wars makes it pretty clear that Finland can defend itself, at least for some time, so that alone is deterrence against Russia invading Finland. Inviting Finland into NATO would then seem mostly an empty gesture (since we all know NATO won't actually fight Russia), but also a provocative action (see Ukraine). It's lose-lose.

It's time for sane thinking to prevail at NATO. The status quo ante-Georgia was fine. The current state of play is not. We got here in large part because of overly-optimistic belief in NATO. Instead of deterring Putin, the inverse happened. The reason is simple: NO ONE believes that NATO would actually fight. Even back in the 80s under Reagan, no one believed any countries other than the U.S. and UK would really fight, but at least Western Europe was defensible and worth fighting for, and the U.S. being willing to fight was deterrence enough. Eastern Europe is now defensible and worth fighting for, and I do believe Poland would fight for itself, and so would other Eastern European countries, but I don't think any would fight for Ukraine, not even if Ukraine had been admitted a few months ago.