This is a question of political and ethical judgment rather than pure fact, so I'll share perspectives on both sides rather than a definitive verdict—it's genuinely contested, and reasonable, well-informed people disagree.
Arguments that NATO has been a force for good:
•Deterrence and collective defense: NATO's Article 5 mutual defense commitment is credited with helping prevent large-scale war in Europe for over 70 years by making aggression against any member costly for the whole alliance.
•Stability and democratic anchoring: Membership has often been tied to democratic reforms and civilian control of the military, which arguably helped stabilize parts of Eastern Europe after the Cold War.
•Response to Russian aggression: Many argue NATO's existence and expansion have been vindicated by Russia's invasions of Georgia (2008) and Ukraine (2014, 2022), providing a unified Western response, sanctions coordination, and military support to Ukraine.
•Burden-sharing for security: It allows smaller/weaker states to pool defense resources rather than each needing to independently develop deterrents (including nuclear ones).
Arguments that NATO has been harmful or is problematic:
•Provocation thesis: Some analysts (realist scholars like John Mearsheimer, and Russian officials) argue NATO's post-Cold War eastward expansion needlessly antagonized Russia, treating a security alliance's growth as inherently threatening to a great power, and contributing to the conditions for conflict in Ukraine.
•Out-of-area interventions: NATO's actions beyond collective defense—like the 1999 Kosovo bombing campaign (conducted without UN Security Council authorization) and the 2011 Libya intervention—are criticized as overreach. Libya in particular is often cited as a case where a UN-mandated "protect civilians" mission expanded into regime change, contributing to a decade of chaos and instability.
•Democratic legitimacy and accountability: Decisions by a military alliance of powerful states can sideline the UN system and smaller states' sovereignty concerns.
•Entrenching militarism: Critics on the left argue NATO perpetuates a Cold War logic of military blocs and high defense spending rather than pursuing collective security through diplomacy or international institutions.
•Double standards: Some point to inconsistency—intervening in some humanitarian crises but not others—as evidence NATO's actions are driven by member-state strategic interests rather than principle.
My honest take
I don't think "good" or "bad" is the right binary. NATO is a tool of collective security for its members, and its value depends heavily on what you think the realistic alternative would have been (a Europe without a unified defense bloc, more unilateral national militaries, possibly more Russian influence) versus what its actions have actually produced in specific cases (Kosovo, Libya, Ukraine support). It has likely prevented some wars among its members and arguably contributed to the preconditions for others outside it.
If you want, I can dig deeper into any specific case (e.g., NATO expansion and Ukraine, Libya, or current debates about NATO's role) with more current reporting and expert analysis.