...to people who neglect the study of history.
NATO - by far the most powerful military alliance ever - changed the perception of the concept of a military alliance radically, and in several ways:
(1) Collective defence
Alliances used to be not strictly defensive; alliance member "A" could declare war on rival "B", B's ally "C" would declare war on A and A's ally "D" would declare war on "C".
NATO was understood to be no such alliance. It was considered to be a collective defence institution, a defensive alliance. That is, till the Yugoslavian Civil Wars.
(2) Alliance organization
Nowadays an alliance doesn't have the feel of an alliance any more if it has no administrative headquarters or no military multinational command headquarters with bunker.
These things are actually novelties introduced by NATO because it inherited the wartime HQ structures of the Western Allied powers. Earlier alliances usually even lacked a common flag or symbol.
|NATO AWACS aircraft, (c) jwh|
(3) What it means to fight an enemy
It used to be like this; a member of the alliance is attacked, the alliance members wage war, peace is being concluded with the aggressor government and only in rare cases the temporarily hostile country would be occupied for a while (till reparations were paid, for example).
Nowadays a member may be attacked, the alliance goes to war with the dudes who gave hospitality to the aggressor's inciter, they destroy this regime, chase regime members out of the country and then wage a crazy 12+ year long war to keep them from ever coming back to power because ... uhmm, nobody figured that out yet. It sure didn't use to be the normal modus operandi of alliances.
(4) What the treaty text means
Treaty texts were central to historical alliances.
Not so with NATO. The North Atlantic Treaty is basically a "and if one gets attacked, the others do about it what they deem necessary" kind of text. The obligations stemming from the text are a joke, and thus the perfect opposite of how seriously (imagined) alliance obligations are being taken.
Basically nobody would have uttered something as "they were our valuable allies for decades, so we need to be thankful to them" prior to the post-Cold War NATO. It was widely understood that alliances (usually between monarchs or princes) were based on mutual benefits, not charity or about accumulating some kind of political credit. A country which didn't need the alliance any more would leave it, period. In fact, powers often left an alliance and joined a rival alliance. Italy even pulled this off during both world wars!
This brings us to the next novelty; few if any alliances of NATO's size were ever considered to be eternal. There were long-lasting alliances, for sure - but usually between an empire and some peripheral small power which served as buffer to barbarians. NATO lasts for generations already, it outlived its original raison d'être and just doesn't seem to have any kind of "exit strategy" for itself. It became a bureaucracy, after all - with survival instinct.
(7) Expedition pool
No alliances (not counting internal relations of the British empire as alliances) served primarily as a pool for expeditions, interventions and small wars prior to post-Cold War NATO. Nor did any previous alliance operate its own air force (I know, the AWACS aircraft fly under the flag of Luxembourg, but everyone knows they're basically NATO aircraft). NATO has become a club which provides support functions and ready-to-go staffs for missions which are utterly unrelated to the alliance treaty (see #4).
(8) Plans to mutilate itself
Never before NATO and the Warsaw Pact did an alliance plan to kill the people of one or several of its members en masse, or even maintained such plans for decades. Both these alliances/pacts had indeed plans about killing millions of their own people with thermonuclear bombs in the event of war just because.
An Oder bridge happens to be at a East German city? That city didn't survive the next wargame. Or the wargame after that. Maybe no theatre-level wargame ever.
No previous alliance was this insane or perverted.
One might be enticed to attribute something outstanding and positive to NATO; peace for decades. This is not a novelty, though: Germany was not involved in a European war (and in only a half dozen expeditionary ones) from 1872-1913, for example.
The study of history makes the extraordinary nature of NATO very visible, but the modern perception of what an alliance is like is clearly focused on NATO reality, not on any precedents.
The Lisbon Treaty's section on collective defence is a serious alliance by pre-NATO standards, while NATO was never really an alliance by pre-NATO standards (too wobbly wording of its treaty).