McMaster is no apologist for Vladimir Putin. He has carefully assessed how the American military might deter and counter Russian provocations. “Historians will likely regard Russia’s 2014 invasion of Ukraine as the event that punctuated the end of the post-Cold War era,” McMaster wrote in a book review for the Wall Street Journal in March 2016. He may very well be right. History has turned on less consequential events.

For McMaster, Russia’s prying Crimea away from Ukraine and annexing it should not be viewed as an isolated act of aggression. Instead, it is one of a series of challenges to American leadership. And it isn’t just Russia that is seeking to shift the balance of power in the 21st century. Regional powers such as China and Iran are expanding their footprints. Russia, China, and Iran may not be capable of mounting a sustained challenge to America’s global leadership on their own. Not yet, that is. But working in parallel, and sometimes through coordinated action, they could tip the scales in favor of anti-democratic, anti-Western forces.

McMaster’s warning with respect to Russia’s invasion of Crimea was based on his reading of The Unquiet Frontier: Rising Rivals, Vulnerable Allies, and the Crisis of American Power, by Jakub J. Grygiel and A. Wess Mitchell. The authors are keenly aware that Americans are questioning the value of our alliances today, more than in the past. And they seek to reestablish the case for American supremacy in the world.