The 53-year-old McMaster was one of those who spent the past decade or so re-orienting the Army away from traditional war-fighting. But he is widely considered one of the service’s top strategic thinkers and his supporters insist he is the best person to figure out how to respond. “He learns and he thinks about what could be and what should be,” says Sullivan, the retired Army chief of staff.

McMaster’s pioneering tactics in confronting the Iraq insurgency after the 2003 invasion were rewarded with a key role under General David Petraeus in rewriting the Army’s field manual on counterinsurgency operations. It was not an easy undertaking. The U.S. military had not focused on counterinsurgency operations in the decades since the war in Vietnam. As a colonel and brigade commander in 2005 in Iraq’s western Al Anbar province, McMaster helped pioneer a strategy that came to be known as “clear, hold, build”—in which swarms of U.S. forces backed by airstrikes secured a city or town and built up the local security forces until they were deemed ready to maintain security while local government institutions could mature.

But getting the Army as an institution to focus on training and buying the necessary equipment to fight bands of terrorists and guerrillas hidden in population centers—instead of big tank formations like the Iraqi Republican Guard it clobbered in the 1991 Persian Gulf War—proved extremely challenging.

The steady erosion of public support for the conflict—and growing angst in Congress about the seeming lack of an end game—didn’t help.

What is taking place in Ukraine, however, is seen as a game-changer. McMaster and the study team he has put together believe their work could have huge impact on what the Army buys, how it trains and how its units are structured for years to come—maybe even as much as the Yom Kippur War did.

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The Army has a long history of trying to learn from wars it didn’t fight—and fold the battlefield lessons into its own arsenal.

A decade before the carnage of the American Civil War, George McClellan, who later became the commander of the Union Army, was an official observer of the European armies engaged in the Crimean War, which Russia lost to an alliance of France, Britain, the Ottoman Empire and Sardinia. That conflict is widely considered the first modern war, in which mass-produced rifles, explosive shells, mines and armored landing craft were first used. John Pershing, who commanded allied forces in World War I had also previously observed the Russo-Japanese War.

But the current thinking of McMaster and his top aides on what the Ukraine war might mean for the U.S. is eerily parallel to the experience of the early 1970s. That is when the U.S. military had been distracted by another guerrilla war, in Vietnam, while Russia’s military grew bolder and more sophisticated, posing a new threat to NATO, the Western military alliance.

It’s not the actual 1973 war that the Army believes parallels the modern-day conflict in Ukraine but rather the Army’s approach afterward in digesting its lessons—and folding them into its own war plans. The study of that earlier war “serves as a useful model for analyzing the conflict in Ukraine,” says Colonel Kelly Ivanoff, a field artillery officer and top aide to McMaster, who adds that the detailed undertaking to study the 1973 war was to “profoundly influence the development of the U.S. Army for the next 15 years.”

The Russia New Generation Warfare study will “examine the Ukraine theater for implications to Army future force development, with emphasis on how Russian forces and their proxies employed disruptive technologies,” he added.

The effort, which is just getting underway, is focused on 20 separate “warfighting challenges”—including maintaining communications in the face of cyberattacks; developing a greater degree of battlefield intelligence; redesigning Army combat formations and tactics; and identifying new air defenses, weapons and ways to employ helicopters.

Indeed, where the Yom Kippur War analogy reaches its limits, say close observers, is the way in which Russia has also employed other, nonmilitary power—first during the Russian military annexation of the Ukrainian peninsula of Crimea in 2014 and then in its ongoing proxy war in eastern Ukraine.

“They looked at what we were doing in the early ’90s and some of what we were saying we wanted to do and went one better,” said Sullivan, who served as Army chief of staff from 1991 to 1995 and now runs the Association of the U.S. Army, an advocacy group. “They started adding the special operating forces, which included diplomats, people who were subverting [the Ukrainian government] from the inside. It’s a hybrid.”

Now, he said, the Army is trying to apply “what we learned about the way they are using their little green men—people who are subverting the governments.”

That is not to say that the Russian Army and its proxies are 10 feet tall. The Ukrainian Army is credited with deterring an all-out Russian invasion. And the briefing that has been shared at the highest levels of the Army and with a number of foreign allies points out that the Russian military shrank dramatically in size between 1985 and 2015. And its biggest weakness is widely considered its conscript army, which has limited training and suffers from poor morale.

General Starry, who led the Yom Kippur War after-action review, concluded that the quality of the soldiers ultimately can carry the day—not numbers. “It is strikingly evident,” he wrote later, “that battles are yet won by the courage of Soldiers, the character of leaders, and the combat experience of well-trained units.”

But combined with Moscow’s efforts to upgrade its nuclear forces, what has been on display in eastern Ukraine and more recently in its military foray into Syria is expected, at least by the generals, to change the U.S. Army for a long time to come.