At a press conference Tuesday, Secretary of Defense Gen. James Mattis was asked by a reporter why the United States responded to Syria's chemical weapons attack, but not to the Assad regime's use of barrel bombs.

The reporter asked why the deaths of civilians by a sarin-type gas warranted a military response, but not barrel bombs, which have killed many more people in the war-torn country.

Barrel bombs are meant to explode just above the ground and send several pounds of shrapnel in all directions for maximum casualties.

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Mattis called President Trump's response to the chemical attack a "policy decision," adding that the United States is limited it what it can do in foreign lands.

"We could not stand passive on this," Mattis said.

He said that Syrian President Bashar al-Assad's use of chemical weapons violates a World War I-era agreement against the use of such weapons against civilians.

Mattis said that fact, combined with Russia's promise that chemical weapons were removed from Syria meant that further use of them would result in payment of "a very stiff price."

He expressed concern at those wanting America to militarily go all-in in what he called the "most complex civil war raging on the planet."

"The intent was to stop the cycle of violence," he said of the 59-missile response.

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