THE BEST DIPLOMACY IS ARMED DIPLOMACY By Henry R. Nau

Since Barack Obama came to the White House, he has practiced an activist diplomacy. The president never met a diplomatic opportunity he did not like—yet he has steadily reduced American power to back up that diplomacy.

The deal with Russia to eliminate Syria’s chemical weapons is just the latest example. It is all diplomacy, while the power to back it up remains halfhearted: a pledge to ask Congress to authorize an “unbelievably small” strike.

“Diplomacy without arms,” the Prussian King Frederick the Great once said, “is like music without instruments.” He meant that unless the adversary is prevented from achieving his objectives by arms outside negotiations, he has no interest in taking seriously peaceful alternatives being offered insidenegotiations.

Take the case of Iran, which is worse than Syria. Today, Iran is achieving its objectives by force while the West negotiates. Tehran marches steadily toward a nuclear capability, fuels jihadist forces in Lebanon and Syria, and expands its influence in Iraq and Afghanistan, filling the vacuum the U.S. has left behind.

Meanwhile, the U.S. contemplates force only if negotiations fail. It cuts its defense budget by a mindless sequestration, scales back missile defenses in Europe—against Iran, among other potential adversaries—with no concessions from Moscow, and leaves no residual force in Iraq that might check Iranian influence. It refrains, up to now, from arming Syrian rebels who are fighting a game-changing civil war, and pivots military forces to the Far East that are now clearly needed in the Mediterranean.

Yet America’s emphasis on diplomacy isn’t the problem. The problem is its failure to arm diplomacy. The despots in Tehran and Damascus use arms daily at home and readily abroad before and during negotiations. If the U.S. uses arms only after negotiations fail, despots use force unopposed until they achieve their objectives.

The use of force—the buildup, deployment and actual use—before and during negotiations serves three purposes. First, it puts the adversary on notice that it is unlikely to compete outside negotiations. Ronald Reagan launched a massive and risky military buildup to signal to the Soviet Union that it could not win an arms race.

Second, force denies the adversary gains on the ground outside negotiations. Reagan pushed back against the Soviet Union by deploying Intermediate Nuclear Force missiles in Europe and supporting freedom fighters in Afghanistan and Central America. And third, the use of force brings heavy-duty capabilities to bear at the bargaining table—in Reagan’s case, the Strategic Defense Initiative, designed to protect the U.S. from ballistic-missile attack.

Arms compel the adversary to take diplomacy seriously. And using lesser force early often avoids the need for greater force later.

When President Truman ordered the famous airlift in 1948 to save West Berlin, even though all of his advisers urged him to abandon this enclave in the Soviet zone, he avoided a much bigger land war in Europe—like the one that followed in Korea where the U.S. failed to use lesser force early. As George Shultz, Reagan’s secretary of state, once said: “it is better to use force when you should rather than when you must; last [resort] means no other, and by that time the level of force and the risk involved may have multiplied many times over.”

The purpose of armed diplomacy, however, is not to defeat adversaries in some conventional military showdown, or to coexist with them indefinitely in some morally ambivalent status quo. It is to succeed in negotiations that move freedom forward in adversary countries. Not just any compromise achieves this objective, but an unwillingness to compromise at all—that is, to trade off military leverage at some point for diplomatic gains—squanders hard-won military leverage.

In February 1848 American forces sat in Mexico City, and many in Congress called for the annexation of all of Mexico. But President James K. Polk knew when to compromise. He accepted the Treaty of Hidalgo Guadeloupe negotiated by a renegade envoy, whom he had fired three months earlier. He traded maximum military leverage to gain his original territorial objectives: present-day California, Utah and New Mexico. By July 1848 not a single American soldier remained in Mexico.

What might such compromises look like in Iran or Syria? First, don’t stop calling despots despotic. Mr. Obama, with his “realism” toward Russia and China, has gone too far in ignoring their human-rights violations. Thomas Jefferson despised the tyrant in Paris and said so—even as he negotiated with Napoleon to secure the Louisiana Purchase.

Second, fashion an outcome that despots can accept in the short-run but does not rescue their sclerotic systems in the long-run. In Syria, that may mean negotiating with Bashar Assad longer than we would like. In Iran, it may mean accommodating a civilian nuclear program with less than perfect inspection guarantees if the country opens up to freer trade and contacts, much the way the Helsinki Accords opened contacts and trade with the former Soviet Union.

But none of these compromises is advisable as part of negotiations without persistent pressures surrounding the negotiations to maintain economic sanctions, protest human-rights violations and checkmate forceful arms interventions on the ground. When armed diplomacy works best, no military force has to be used. But it is a mistake to assume that such military force isn’t needed.

Mr. Nau is professor of political science and international affairs at George Washington University and author of “Conservative Internationalism: Armed Diplomacy under Jefferson, Polk, Truman, and Reagan” (Princeton, 2013).