As United Nations inspectors assess whether either side used chemical weapons in an attack that killed hundreds of people in Syria’s civil war, there is a heated debate in this country over President Barack Obama’s authority to use military force. Well over 100 House members signed a letter saying Obama must have congressional approval.

"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 said.

The latest poll suggests the public leans heavily the same way. According to NBC polling, nearly 80 percent of Americans think Obama should get the go-ahead from Congress before launching any attack.

Two key members of the president’s Cabinet, Secretary of State John Kerry and Secretary of Defense Chuck Hagel, have been talking to congressional leaders, trying to make the case that facts on the ground show that the Syrian government deployed a chemical attack and should pay the consequences.

The country has wrangled over whether the president can send American forces into action without the approval of Congress before, most recently in dealing with Libya. Here is what our previous work tells us about the legal and political framework regarding Syria.

The key laws

The Constitution and the War Powers Resolution set the terms for military engagement. In Article II, Section 2, the constitution says the president is commander in chief of the armed forces. Then, in Article I, Section 8, it assigns the right to declare war to Congress.The last time that actually happened was at the beginning of World War II, when Franklin Roosevelt was president.

Since then, presidents have generally initiated military activities using their constitutionally granted powers as commander in chief without having an official declaration of war in support of their actions.

In 1973, in frustration with the Vietnam War, Congress passed the War Powers Resolution, which was enacted over a veto by President Richard Nixon. The resolution required that, in the absence of a declaration of war, the president must report to Congress within 48 hours of introducing armed forces into hostilities and must terminate the use of U.S. armed forces within 60 days unless Congress permits otherwise.

During the Libyan conflict, Walter Dellinger -- former head of the Justice Department’s Office of Legal Counsel and acting solicitor general under President Bill Clinton -- wrote that by regulating the principle "that the president may introduce troops into hostilities or potential hostilities without prior authorization by the Congress," the War Powers Resolution effectively confirms that the president can act unilaterally.

Other experts we asked generally agreed with this line of argument. In short, the president can send in forces, but he or she can’t keep them there unless Congress approves.

Precedent matters

The president might have the authority to use military force, but we found many significant examples when the White House sought congressional approval. Lawmakers passed resolutions allowing military action when President Ronald Reagan sent marines to Lebanon in 1983, when Iraq invaded Kuwait in 1991, and for the wars in Afghanistan and Iraq.

With the humanitarian mission to Somalia in 1992, the President Georg H. W. Bush acted unilaterally to send troops but when Clinton took office, he asked for a resolution. The Senate and House passed separate measures approving the deployment, although the two versions were never reconciled and were never formally adopted.

In general, presidents acknowledge the War Powers Resolution without accepting that they are bound by it. American participation with NATO forces in Kosovo in 1999 generated resistance, particularly in the U.S. House.

As a legal matter, there was a question whether the War Powers Resolution applied when cooperation with NATO was involved. U.N. resolutions can raise similar issues.

Clinton notified Congress as the law required but U.S. military action continued past the 60-day limit without congressional approval. A group of congressmen sued to have the courts rule that the president had violated the War Powers Resolution but the courts declined, saying the representatives did not have standing to sue.

When Obama moved to support NATO attacks in Libya, he did not seek congressional approval. In fact, the White House argued that technically, the U.S. was not engaged in hostile activities. The administration’s case was based on the absence of "boots on the ground" or war planes conducting direct attacks.

We found that while prior administrations had taken similar approaches as the Obama administration in analogous situations, that didn’t mean that the argument was legally justified.

How direct a threat

The question of whether the country faces an immediate threat plays a pivotal role in the political debate. In the case of Libya, Obama argued that, "Left unaddressed, the growing instability in Libya could ignite wider instability in the Middle East, with dangerous consequences to the national security interests of the United States."

Invoking the idea of indirect threats has always been open to strong disagreement. The House members who warned against an attack on Syria without congressional approval stated explicitly, that "no direct threat to the United States exists".

In our past coverage, the matter of defining the country’s stake when intervening overseas was not a legal dispute, but one based in politics and policies. We found that as a candidate and as president, Obama himself had made opposing legal arguments for military intervention.

Our past coverage

We tackled the question of the scope of the War Powers Resolution in this article about the Obama administration’s argument that the law did not apply in Libya because the U.S. was not engaged in hostilities. We explored some interesting legal parsing of language and the gap between what the law says and how it is interpreted. Read more here .

When Rep. Charlie Rangel, D-N.Y., said that Roosevelt was the last president to seek congressional permission for war, we looked into the history. While technically, Rangel is right, we noted that at least six presidents have sought congressional approval for military action. Learn more about the history here.

America’s effort to remove Muammar Gadhafi from power raised the claim that Obama had flip-flopped from the position he held in 2007. During an interview, then-candidate Obama 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."