During the Cold War, when the U.S. and the USSR were the only two superpowers, or after it, when the U.S. was the sole superpower, calculations of power were easy. With only two players, communication is easier, misperception is harder and strategic calculations are easier to execute. There is one enemy, the pieces are set, and there is less room for error.
Enter the modern multipolar world. With more than two players, calculations of power are harder. Shifting alliances make assessment of relative strength harder still. For example, in worlds of five great powers, what is the balancing point where alliances become equipotent? What if one country decides to change sides because its leaders think that as part of a stronger alliance it will be able to eliminate the remaining two and take the spoils of war? Now add a twist — what if this calculation is wrong, and the relative assessment of power does not favor it? You get the picture.
As a real, modern example, consider the U.S., Europe, Russia and China. The U.S. is the leading power. Europe is a big economic power and has a large population, but not a united army. China is fast ascending and its interests are in the pacific, and so are, increasingly, that of the U.S. Russia is in the worst shape, with a declining population and moderate force, but geography that favors it, and with immediate neighbors that are divided and do not match it in strength, a fact it has adroitly exploited recently (see: Ukraine, Georgia, but also the Baltics).
Now lets say that Trump, as he’s said he’d do, pulls out of NATO and Europe, and lets say he strikes a deal with Russia to ally against China. The tradeoff is Eastern Europe and the Baltics and maybe concessions in the Middle East. In the absence of an alliance framework (NATO will now be defunct in Europe) and shifting power what does Europe do? Side with Putin and concede influence over the Northern Sea, the Baltics and implicitly open the door to Eastern Europe? Or does Brussels decide Europe’s vital interests are threatened and go to war over these areas? In that event, how does China respond? Is it tempted to strike Russia while it is weak and focused on its western front, or is it sufficiently tied up in the Pacific by the U.S. to keep it from doing that. Or maybe Beijing thinks that it can handle two fronts? How does the U.S. respond?
Of course this is grand strategy and to reach that point there are millions of decisions at the bureaucratic level, many rounds of diplomacy and multiple rounds of military escalation. But all these minute decisions have one overriding factor that drives them: they assess and respond to perceptions of power. And perceptions matter because when power is misperceived then it tempts tests of strength. That means that when calculations are complex, because of the multiplicity of powers, then misperception, and war, is more likely.
If you want to see what happens when many powers jostle for position by shifting alliances and periodic war, look at the history of Europe.