Asked about statements from the Russians and US President Donald Trump regarding "red lines" and threats of all out war, Mattis spoke unequivocally.
"It will not spiral out of control," said Mattis fears of military escalation between Russia and the US over Syria.
"As you know, Secretary Tillerson is in Moscow. We maintain communications with the Russian military and with the diplomatic channels. It will not spiral out of control," Mattis continued.
When pressed by the questioner on how he was so confident that Russia would not follow through with its threats of reprisal if the US strikes Syria again, Mattis stayed on message.
"I'm confident the Russians will act in their own best interests, and there's nothing in their best interests to say they want this situation to go out of control," said Mattis.