XP

If your sole purpose of existence is based around something, you'll probably never want that thing to disappear. For example, if you're a doctor, the day all sicknesses vanish would be the day you lost your job. Although most doctors would probably accept that kind of tradeoff, there will always be those that would prioritize protecting their own (financial) wellbeing.

If the countries within an alliance were to lose a common enemy (like for example a small countries that have banded together to resist a larger one), the alliance would sooner or later dissolve. Without alliances, the likelihood of a war breaking out rises drastically.