Key point: America is strong enough that no one would try it, but no world war would be worth the cost.
The United States discarded its oft-misunderstood “two war” doctrine, intended as a template for providing the means to fight two regional wars simultaneously, late last decade. Designed to deter North Korea from launching a war while the United States was involved in fighting against Iran or Iraq (or vice versa,) the idea helped give form to the Department of Defense’s procurement, logistical and basing strategies in the post–Cold War, when the United States no longer needed to face down the Soviet threat. The United States backed away from the doctrine because of changes in the international system, including the rising power of China and the proliferation of highly effective terrorist networks.
But what if the United States had to fight two wars today, and not against states like North Korea and Iran? What if China and Russia sufficiently coordinated with one another to engage in simultaneous hostilities in the Pacific and in Europe?
Could Beijing and Moscow coordinate a pair of crises that would drive two separate U.S. military responses? Maybe, but probably not. Each country has its own goals, and works on its own timeline. More likely, one of the two would opportunistically take advantage of an existing crisis to further its regional claims. For example, Moscow might well decide to push the Baltic States if the United States became involved in a major skirmish in the South China Sea.
In any case, the war would start on the initiative of either Moscow or Beijing. The United States enjoys the benefits of the status quo in both areas, and generally (at least where great powers are concerned) prefers to use diplomatic and economic means to pursue its political ends. While the U.S. might create the conditions for war, Russia or China would pull the trigger.