After World War II, Sen. Arthur Vandenberg, a Michigan Republican who was chairman of the Foreign Relations Committee, formed a bipartisan partnership with President Truman that helped secure the postwar peace and greatly strengthened America’s position in the Cold War. “Politics stops at the water’s edge,” said Vandenberg when asked why he worked so closely with a Democratic president. He added that his fellow Americans undoubtedly had “earnest, honest, even vehement” differences of opinion on foreign policy, but if “we can keep partisan politics out of foreign affairs, it is entirely obvious that we shall speak with infinitely greater authority abroad.”

In their uniformly skeptical or negative reactions to Soleimani’s death, Democrats are falling well below Vandenberg’s standard and, I fear, creating the risk that the U.S. will be seen as acting and speaking with less authority abroad at this important time.

No American can dispute that Soleimani created, supported and directed a network of terrorist organizations that spread havoc in the Middle East. In Syria he made it possible for the Assad regime to respond with brutality to its own people’s demands for freedom. More than 500,000 Syrians have died since 2011 and millions more have been displaced from their homes.

During the Iraq war, Soleimani oversaw three camps in Iran where his elite Quds Force trained and equipped Iraqi militias. According to the U.S. government, these fighters have killed more than 600 American soldiers since 2003. In another time, this would have been a just cause for an American war against Iran, and certainly for trying to eliminate Soleimani. Within Iran, the Quds Force has worked with the supreme leader to suppress freedom and economic opportunity, jail dissident politicians and journalists, and kill protesters in the streets.

From the perspective of American values and interests, it’s impossible to mourn the death of such a man, and Democrats haven’t. Their response thus far has been “Yes, but . . .,” adding worries that Soleimani’s death will provoke a violent response from Iran. Democrats have also suggested that the Trump administration has no coherent strategy toward Iran or that Mr. Trump shouldn’t have acted without notice to and permission from Congress.

Yet if we allow fear of a self-declared enemy like Iran to dictate our actions, we will only encourage them to come after us and our allies more aggressively. Some Democrats have said that killing Soleimani will lead us into war with Iran. In fact, Soleimani and the Quds Force have been at war with the U.S. for years. It is more likely that his death will diminish the chances of a wider conflict because the demonstration of our willingness to kill him will give Iranian leaders (and probably others like Kim Jong Un ) much to fear.

Some Democrats have also refused to appreciate Soleimani’s elimination because they say it isn’t part of an overall strategy for the region. But based on the public record, there is a strategy, beginning with the Trump administration’s withdrawal from the Iran nuclear agreement, the shift to maximum economic pressure, and now adding a demonstrated willingness to respond with military force to Iran’s provocations. The goal is to bring the Iranian government back into negotiations to end its nuclear weapons program and rejoin the world’s economy.

The claim by some Democrats that Mr. Trump had no authority to order this attack without congressional approval is constitutionally untenable and practically senseless. Authority to act quickly to eliminate a threat to the U.S. is inherent in the powers granted to the president by the Constitution. It defies common sense to argue that the president must notify Congress or begin a formal process of authorization before acting on an imminent threat.

On many occasions President Obama sensibly ordered drone strikes on dangerous terrorist leaders, including U.S.-born Anwar al-Awlaki. He did so without specific congressional authorization, and without significant Democratic opposition. Mr. Obama also “brought justice” to Osama bin Laden without prior, explicit congressional approval.

It is possible that anti-Trump partisanship isn’t behind Democrats’ reluctance to say they’re glad Soleimani is dead. It may be that today’s Democratic Party simply doesn’t believe in the use of force against America’s enemies in the world. I don’t believe that is true, but episodes like this one may lead many Americans to wonder whether it is. If enough voters decide that Democrats can’t be trusted to keep America safe, Mr. Trump won’t have much trouble winning a second term in November. That’s one more reason Democrats should leave partisan politics at “the water’s edge” and, whatever their opinion of President Trump on other matters, stand together against Iran and dangerous leaders like Qasem Soleimani.