Mastanduno, however, does conclude that eventually power will balance power. I would like to look at the question from another angle. Balances of power do not form quickly. For example, a balance of power against Hitler did not form until 1941-1942, years after the war had begun, and after the United States had entered the war on the side of the Allies. States only balance when they must.
Balances are difficult to form and costly.
States are often tempted to jump on the bandwagon and appease instead of fighting a war. Going against a country that looks like it may be the winner will cost a lot of arms, effort, and lives. Secondly, earlier great wars left enough major powers standing to provide the materials for forming a new balance of power. In a bipolar world, if one of the countries representing one of the poles disappears, by definition there is only one great power left. If that one great power is going to be balanced, it requires some other country lifting itself to the level of that one great power.