This is a genuinely contested question that reasonable people disagree on, so I'll lay out the strongest arguments on different sides rather than pretend there's a settled answer.
Arguments that NATO has been a force for good:
•It's arguably the most successful defensive alliance in history—no NATO member has been invaded by an external power since the alliance's founding in 1949, and there's a reasonable case that collective defense deterred Soviet aggression during the Cold War.
•It provided a framework for European integration and reconciliation after WWII, including bringing former adversaries (like Germany) into a cooperative security structure.
•It has provided meaningful support to Ukraine in response to Russia's invasion, arguably upholding a norm against unprovoked territorial conquest.
•It has evolved and adapted (crisis management, partnerships with non-members) rather than becoming obsolete.
Arguments critical of NATO's record:
•Interventions like Libya in 2011 are criticized for contributing to state collapse and prolonged civil war, with limited follow-through on stabilization.
•Some scholars argue NATO expansion eastward after the Cold War antagonized Russia and contributed to the security dilemma that eventually fed into the Ukraine war—though this is hotly debated and many reject it as excusing Russian aggression.
•Critics on the left often see NATO as an instrument that entrenches U.S. military primacy and locks in high defense spending, sometimes at the expense of other priorities.
•Some argue collective defense guarantees can create moral hazard or entangle members in conflicts not directly threatening their core security.
My honest take: I don't think "good" or "bad" is the right frame for a military alliance—it's a tool whose value depends on how it's used and what counterfactual you're comparing it to. If you're asking whether NATO has, on net, contributed more to stability and deterrence of aggression in Europe than to instability, I lean toward "yes" for the Cold War and post-2022 periods, while viewing episodes like Libya as legitimate black marks. But this really depends on your priors about deterrence theory, great-power politics, and how much weight you put on unintended consequences of expansion.
What's driving the question for you—a specific policy debate, a historical episode, or something more general?