A first lesson is that well-meaning treaties, international law and the like are not enough to keep the peace or protect U.S. interests.

Wilson realized this, albeit far too slowly. It pays to recall that when German U-boats began attacking merchant ships, Wilson said he would hold the Kaiser to “strict accountability.” Yet when Americans were killed aboard the Falaba, Lusitania and Arabic, he responded by writing letters.

President Theodore Roosevelt, who became something of a one-man shadow government in the years between the onset of hostilities and U.S. entry into the war, put it well in 1914: “In time of crises, peace treaties are worthless.”

He hammered that point repeatedly during the war, pointing out the “utter worthlessness of treaties” and how they “offer not even the smallest protection against such disasters.”

Importantly, these words come from a man who believed in diplomacy, a man who negotiated important treaties that staved off and ended wars in Europe, Africa and the Asia-Pacific, a man who earned a Nobel Peace Prize for his diplomatic efforts. But years of experience had taught him that “diplomacy is utterly useless where there is no force behind it.”

A century later, this truth remains unchanged, because man’s nature remains unchanged: Bad guys do bad things. A piece of paper, a presidential address, a sternly worded communique, a UN resolution, a portfolio of “targeted sanctions” seldom correct or prevent bad behavior. None of these has stopped Putin from annexing Crimea or waging war against Ukraine; or China from trying to incorporate Philippine, Japanese and Vietnamese territories and waters; or North Korea from firing missiles or testing nukes; or jihadists from kidnapping young women in Nigeria or tearing Iraq apart; or Assad from bludgeoning his people; or Iran from continuing its drive for nuclear weapons. (The nonpartisan Congressional Research Service concludes, “The principal objective of international sanctions—to compel Iran to verifiably confine its nuclear program to purely peaceful uses—has not been achieved.”)

On the other hand, diplomacy backed by the threat of force can be highly effective: The very real threat of force convinced Assad to hand over his chemical weapons—or perhaps better said, convinced Putin to pressure Assad into handing them over. Libya’s Moammar Gadhafi did likewise with his vast WMD program in 2003, after he became convinced he would meet the same fate as Saddam Hussein. President Ronald Reagan’s buildup led to the first treaty that eliminated an entire class of nuclear weapons, then to reductions of strategic and conventional weapons, then to victory in the Cold War. A quarter-century earlier, President John Kennedy reinforced his diplomatic efforts during the Cuban Missile Crisis by lofting 90 nuclear-armed B-52s into round-the-clock orbits over the Atlantic, readying 1,300 long-range bombers for immediate takeoff and dispatching 60 warships to the waters around Cuba. Khrushchev got the message.

That leads us to a second lesson from the Great War: Military preparedness can go a long way to keeping the peace, while unpreparedness invites danger.

Americans were neutral noncombatants until April 1917, but were targeted and killed during the Great War. TR noted that during his presidency—the era of the big stick and the Great White Fleet—“not a shot was fired at any soldier of a hostile nation by any American soldier or sailor, and there was not so much as a threat of war.”

“The United States has never once suffered harm because of preparation for war,” TR matter-of-factly explained. “But we have suffered incalculable harm, again and again, from a foolish failure to prepare for war.”

Being prepared for war also is cheaper than waging war. In the eight years before entering World War I, the United States devoted an average of 0.7 percent of GDP to national defense. During the war, U.S. defense spending spiked to 16.1 percent of GDP.

TR detailed the consequences of inadequate defense spending in late 1914: “Our navy is lamentably short in many different material directions. There is actually but one torpedo for each torpedo tube,” he wrote. “For nearly two years, there has been no fleet maneuvering…We are many thousands of men short in our enlistments.”

America returned to its old ways after World War I. In the decade before the attack on Pearl Harbor, the United States spent an average of 1.1 percent of GDP on defense annually. 

Applying the lessons of deterrence—finally—Americans spent an average of 7 percent of GDP on defense during the Cold War to keep the Red Army at bay. It worked.

Yet it seems we have forgotten those hard-learned lessons. Defense will account for 3.2 percent of GDP in 2015, and if current trends hold America will be investing a scant 2.8 percent of GDP on defense by 2023.

“The United States never learns by experience but only by disaster,” TR observed in 1914. “Such method of education may at times prove costly.”

A third lesson of the Great War is that trade isn’t a failsafe inoculation against war.

German iron-ore imports from France grew 60-fold in the 13 years before the war. In 1914, Britain accounted for more than 14 percent of Germany’s exports. These trade links did nothing to address other factors: Germany’s desire for a slice of Europe’s colonial pie, distrust among European powers, territorial disputes, secret security guarantees, the disconnect between democratic and authoritarian states.

As before the Great War, there is a rising authoritarian power that feels hemmed in and entitled (see China); strategic uncertainties abound (consider the question marks created by Washington’s stand-off foreign policy and hands-off approach to crises in Europe, the Middle East and Africa); territorial claims remain unsettled (see the South and East China Seas); and the scope of treaty commitments is uncertain (note the anxiousness of the Philippines and the Baltics).

To be sure, trade is booming in Asia: Japan-China trade is $334 billion annually; U.S.-China trade is $562 billion annually; China accounts for 13 percent of the Philippines’ total trade. Yet Japanese Prime Minister Shinzo Abe sees his nation and China in a “similar situation” to Britain and Germany on the eve of World War I. Kevin Rudd, former prime minister of Australia, draws parallels to prewar Europe in the South China Sea—a region “riven by overlapping alliances, loyalties and hatreds,” and simmering with a mix of “primitive…nationalisms” and “great-power politics.” All the while, U.S. defense spending is ebbing to levels not seen since the interwar years.

Finally, when applying lessons from the Great War, we should take care to draw the right lessons.

A common refrain is that Europe’s arms race triggered World War I. If this were true, then a) there shouldn’t have been a World War II, since the Allies allowed their arsenals to atrophy after World War I, and b) there should have been a World War III, since the United States and the Soviet Union engaged in an unprecedented arms race.

The reality is that miscalculation lit the fuse of World War I. The antidote, as we have learned in the intervening century, is clarity plus strength.

Arms alone aren’t enough to deter war. After all, the great powers were armed to the teeth in 1914. But since they weren’t clear about their treaty commitments, a small crisis on the fringes of Europe mushroomed into a global war. Nor is clarity alone enough to deter war. After all, Wilson’s words to the Kaiser were clear, but America lacked deterrent military strength.

We can hope that America has enough residual muscle to maintain the balance, that America begins to speak with sufficient clarity to prevent miscalculation, that trade ties prevent a great war in the Asia-Pacific. But the signs are not good: Joint Chiefs Chairman Gen. Martin Dempsey expects “the risk of interstate conflict in East Asia to rise.” And historian Robert Kagan ominously warns, “The United States and China are no more dependent on each other’s economies today than were Great Britain and Germany before World War I.”