Two kinds of security constellations are imaginable among great powers: the classic balance of power (Realism) or cooperation in the form of collective security (Liberalism). This article posits that the latter has more chances to prevent wars than the former. The case study that is developed is the relationship between Russia and the West after 1989. The West failed to integrate Russia (on an equal footing) in the Euro-Atlantic security architecture after the Cold War. NATO did not only remain in existence; it also expanded on a regular basis, with the promise in 2008 to include Georgia and Ukraine. The result was the continuation of the balance of power game between Russia and the West, finally ending up with the war in Ukraine, something that could have been predicted on the basis of the theory, and that was actually predicted by experts like George Kennan already in the 1990s.