- Expanding membership of a mutual-defense alliance is risky. If you let someone in who's good at picking fights, you can end up in a situation that you really

- Expanding membership of a mutual-defense alliance is risky. If you let someone in who's good at picking fights, you can end up in a situation that you really didn't want to, and that may not have happened in the first place if they hadn't had the treaty to make them feel more secure with e.g. brinksmanship. - Shifts in political alignment of members can, similarly, do strange things and lead to bad situations. Take Hungary—right now they just look maybe just a tad down the path toward authoritarianism, but what if that turned into a civil war? What if the government that you're allied with gets into a bit of the good ol' genocide? Serb/albanian type situation, or maybe just your run of the mill widespread political purges? On the one hand there's some moral obligation to intervene, plus the legitimate interests of neighboring states in countering severe violence and instability in their vicinity; on the other hand now you and anyone else who's considering intervention, in the alliance or not, has a bit of a dilemma on their hands. It complicates an already complicated situation. - As an alliance grows stronger it invites reaction from opponents. This doesn't make those reactions "right" (I'm very aware that this is one argument for why Russia invaded) but it's a fact. The reaction needn't be war, but might be creation or expansion of opposing alliances. Larger alliances, more shared borders in more places... that's a recipe for world war. Every member you add is, in a sense, a long-term liability and threat to the stability of the alliance. These arrangements are not purely beneficial, in practice, including for members.