Mr. Hennigan writes about national security for Opinion.

The U.S. military was once a tool of last resort for American presidents.

The tough decision to deploy armed troops for conflict in another country came after diplomacy, political pressure and other peaceful options were exhausted. This clearly doesn’t hold true in the second Trump administration.

In a short video posted on Saturday morning, President Trump stood in a darkened room at a lectern in a white “U.S.A.” hat and announced that the United States military had begun “major combat operations in Iran” and called for the overthrow of its government. He warned that this could be a costly fight, and that American lives could be lost.

In his attempt to project power to all corners of the globe during his second term, Mr. Trump has routinely relied on both the threat and the use of military action to coerce opponents and allies alike into giving him what he wants. Warnings of American military involvement — traditionally viewed by commanders in chief as “break glass in case of emergency” — have now become a weekly, if not daily, occurrence.

The world has watched Mr. Trump launch military operations in Iran, Yemen, Somalia, Nigeria, Iraq and Syria; capture and remove Venezuela’s leader, Nicolás Maduro, from power; threaten to use military force to take control of Greenland; pressure Mexico to allow U.S. troops in the country to target fentanyl labs; and direct an air campaign to kill suspected drug smugglers at sea.

If you asked average Americans what the mission in Iran is for, you’d likely get conflicting answers. And you couldn’t blame them: The president himself has given several reasons to justify the historic attack now underway.

“The United States military is undertaking a massive and ongoing operation to prevent this very wicked, radical dictatorship from threatening America and our core national security interests,” Mr. Trump said. “We’re doing this for the future, and it is a noble mission.”

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W.J. Hennigan writes about national security, foreign policy and conflict for the Opinion section.

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