Credit: Evan Vucci/AP Photo

Bipartisan pressure is mounting for President Obama to seek congressional authorization for any military strike against Syria, but history has shown that presidents - including Obama - have been willing to forgo formal approval of lawmakers on Capitol Hill before taking military action.

The constitution gives Congress the power to declare war, but Congress has not formally declared war since World War II. The U.S. operations in Korea, Vietnam, Afghanistan, Iraq and Libya were all conducted without any formal declaration of war.

The decision to bypass Congress on military action started with President Harry Truman in 1950 when he sent U.S. forces into Korea. Truman defended his decision by saying it aligned with the recommendations of the United Nations.

When the U.S. intervened in Libya in 2011, the Obama administration justified its decision to not request congressional approval beforehand by citing the 1973 War Powers Act, which allows the administration to conduct military activities for 60 days without first seeking a declaration of war from Congress.

Obama informed Congress of his actions two days later in a letter, saying the decision was "pursuant to my constitutional authority to conduct U.S. foreign relations and as Commander in Chief and Chief Executive" and that he wanted "to keep the Congress fully informed, consistent with the War Powers Resolution." When those 60 days expired, the president wrote a letter to Congress explaining that the U.S. was only playing a supportive role in Libya so he would not need congressional approval to move forward.

President Obama has not indicated whether he will ask for Congress' approval before a potential military strike, but the White House has said the administration is consulting with Congress on the situation. Administration officials will hold an unclassified briefing with key lawmakers on Capitol Hill on Thursday evening to update them on the developing situation in Syria. House Speaker John Boehner was briefed by the president by telephone on Thursday, a spokesman for Boehner said.

"During the call, the speaker sought answers to concerns outlined in his letter to the president yesterday, including the legal justification for any military strike, the policy and precedent such a response would set, and the objectives and strategy for any potential action. Only the president can answer these questions, and it is clear that further dialogue and consultation with Congress, as well as communication with the American public, will be needed," said Brendan Buck, spokesman for Boehner.

Stay up to date with the latest ABC News coverage of the Crisis in Syria.

But should the president decide to pursue an attack without congressional approval, President Obama's actions could run counter to a statement he made in 2007 in which he said, "the president does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation."

And Vice President Joe Biden made a similar claim as a presidential candidate when he said if President George W. Bush attacked Iran without Congressional approval, it would be an impeachable offense.

"The president has no authority to unilaterally attack Iran, and if he does, as Foreign Relations Committee chairman, I will move to impeach," Biden said in 2007.

More than 100 Republican and Democratic lawmakers signed on to a letter Wednesday calling on the president to seek Congress' approval before moving forward with a strike.

"Engaging our military in Syria when no direct threat to the United States exists and without prior congressional authorization would violate the separation of powers that is clearly delineated in the Constitution," the letter read. "If you deem that military action in Syria is necessary, Congress can reconvene at your request. We stand ready to come back into session, consider the facts before us, and share the burden of decisions made regarding U.S. involvement in the quickly escalating Syrian conflict."