"We cannot let chemical attacks go without an answer, and certainly this was an answer," the California Democrat told CNBC's "Squawk Box," referring to the weekend U.S., British and French airstrikes on Syria.
Garamendi said he's introducing legislation that would block any U.S. military actions in Syria without congressional approval.
"It's an answer for which there is no legality. This is an attack on a foreign country, an act of war, and therefore Congress has to declare war or authorization to use military force," he said.
In a later briefing from the Pentagon, Defense Secretary James Mattis said the president ordered the strikes to destroy Syria's chemical weapons development infrastructure, and the U.S. response was a legitimate and proportionate response to Assad's continued use of unconventional warfare.
Garamendi, who was briefed on the apparent chemical attack in Syria by Mattis, said the administration needs to form a strategy to ensure something like the chemical weapons attack doesn't happen again.
"What we have to have here is a coherent strategy based upon a set of objects, hopefully dealing with a reduction in the violence, dealing with the refugees and dealing with the fact that Assad is likely to be there for some time to come," he said.
Ryan Crocker, a former U.S. ambassador to Syria, told CNBC a military response was the right signalm but now, the question for Trump is to figure out how he's going to make a difference.
"This is an inflection point in which we did respond to a chemical attack. So, it's a moment to assess how we move forward," said Crocker, who served as Syria ambassador in Bill Clinton's administration. He also served in roles for Presidents George W. Bush and Barack Obama.
Crocker said the U.S. needs to realize Assad will likely not be leaving anytime soon. "It's good to reflect. We're about 100 years into the new Middle East," he said.
The U.S. is expected to announce Monday more sanctions on Russia because of its support for Assad.
Trump tempered his preference for a more robust attack on Syria after getting a caution from his Pentagon chief, The Wall Street Journal reported. One military option reportedly involved three times the size of the one carried out by forces.