The Obama administration is at last showing some welcome toughness toward Iran, both in words and deeds. Russia is predictably pushing back.

Perhaps in direct answer to Ayatollah Ali Khamenei, who last Thursday set new limits on a proposed nuclear accord strongly pushed by President Barack Obama, Defense Secretary Ashton Carter on Friday told CNN that the administration is keeping open the threat of using military power to set back Iran’s nuclear ambitions. Secretary Carter specifically referred to a recently improved conventional bomb that can successfully target underground facilities.

Also, the Pentagon revealed that it is actively helping Saudi Arabia’s military campaign against the Iran-backed Houthi rebels in Yemen and cooperating in a multi-national Arab naval blockade to prevent Iranian weapons from reaching the rebels.

Both initiatives should help reassure nervous governments from Israel to Egypt to Saudi Arabia that the United States is not passively accepting Iranian nuclear conditions or efforts to expand its power in the Middle East.

Referring to the military option against Iran, Mr. Carter told CNN, “We have the capability to shut down, set back and destroy the Iranian nuclear program and I believe the Iranians know that and understand that.”

The secretary said the administration’s objective is to stop Iran from developing a nuclear weapon through negotiations, rather than through military action.

But he added that the success of the negotiations would depend on verification through a thorough inspection regime, something Ayatollah Khamenei appeared to rule out.

During the interview he also mentioned the Pentagon’s most powerful ground-penetrating bomb, the 30,000 pound Massive Ordinance Penetrator (MOP). The Wall Street Journal reported that in February the Air Force says it has successfully upgraded that weapon and its guidance electronics — and now believes that it could successfully be used to destroy Iran’s secretive underground nuclear facility known as Fordow. The bomb has been tested from a high-flying B-2 stealth bomber. The Journal further reported that Israel has been kept informed of the improvements to the so-called “bunker buster.”

Perhaps in answer to Secretary Carter’s threat, Russian President Vladimir Putin on Monday lifted his government’s ban on selling Iran an advanced air defense system Tehran has long sought.

The Associated Press reported that Russia will offer Iran its top-of-the-line Antey-2500 system, known in NATO as the SA-23. Russia claims that this system is superior to the U.S. Patriot in the number and small size of targets it can engage, its altitude range and speed. It is designed to be capable against cruise missiles and some ballistic missiles as well as aircraft. Russia has already sold a version of this system to Venezuela and offered it to Egypt.

The Russian move is a reminder that Moscow and Tehran have similar disruptive aims in the Middle East.

Whatever the outcome of the Iran nuclear negotiations, the Obama administration must be prepared to protect our friends and interests there.

This latest sign of U.S. resolve — and realism — regarding Iran’s aims is a welcome development.