Combined military strikes against Syria by the United States, British, and French forces involved more targets and twice as many weapons as a similar attack launched almost exactly a year ago, Secretary of Defense James Mattis said late Friday night.

Mattis told Pentagon reporters that the action was necessary because Syrian President Bashar al-Assad had not been deterred from using chemical weapons against rebel military forces seeking to overthrow him and civilian populations in areas controlled by the insurgents.

“Clearly, the Assad regime did not get the message last year. This time, our allies and we have struck harder,” said Mattis. “Together, we have sent a clear message to Assad and his murderous lieutenants that they should not perpetrate another chemical weapons attack for which they will be held accountable.”

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Mattis earlier in the week had said he had not seen conclusive evidence that Assad was behind a chemical weapons attack last week near the city of Douma. He said Friday that he saw that evidence only on Thursday. Those weapons included chlorine and possible sarin gas.

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“I am confident the Syrian regime conducted the chemical attack on innocent people in this last week — yes, absolutely confident of it,” he said.

Marine Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, told reporters that U.S., French and British forces zeroed in on three targets:

A military facility in the greater Damascus area that the regime used for research and development, production, and testing of chemical weapons.

A chemical weapons storage facility west of the city of Homs, located north of the Syrian capital. “We assessed that this was the primary location of Syrian sarin and precursor production equipment,” said Dunford.

A nearby storage facility and command post.

“Important infrastructure was destroyed, which will result in a setback for the Syrian regime,” Dunford said. “They will lose years of research and development data, specialized equipment, and expensive chemical weapons precursors.”

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Mattis said he had no early reports of U.S. casualties but promised details in a morning briefing Saturday when more information would be available.

Dunford said military planners used normal “deconfliction channels” to alert Russia as to what airspace the America military would be using. Both sides routinely share such information to avoid entanglements in a country in which both militaries operate.

But he said the United States did not tip off Russia about the attacks or the targets.

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Mattis said military planners made efforts to avoid civilian casualties.

“We are very much aware this is difficult to do in a situation like this, especially when the poison gas that Assad had assured the world he had gotten rid of obviously still exists,” he said.

Still, Mattis said, Friday’s action represents a stronger response than last year.

“We were not out to expand this,” he said. “We were very precise and proportionate. But at the same time, it was a heavy strike.”

PoliZette senior writer Brendan Kirby can be reached at [email protected]. Follow him on Twitter.