In the space of a single primetime address on Wednesday night, Barack Obama dealt a crippling blow to a creaking, 40-year old effort to restore legislative primacy to American warmaking - a far easier adversary to vanquish than the Islamic State. Obama’s legal arguments for unilaterally expanding a war expected to last years have shocked even his supporters.

Ahead of Wednesday’s speech the White House signaled that Obama already “has the authority he needs to take action” against Isis without congressional approval. Obama said he would welcome congressional support but framed it as optional, save for the authorisations and the $500m he wants to use the US military to train Syrian rebels. Bipartisan congressional leaders who met with Obama at the White House on Tuesday expressed no outrage.

The administration’s rationale, at odds with the war it is steadily expanding, is to forestall an endless conflict foisted upon it by a bloodthirsty legislature. Yet one of the main authorities Obama is relying on for avoiding Congress is the 2001 wellspring of the war on terrorism he advocated repealing only last year, a document known as the Authorization to Use Military Force (AUMF) that few think actually applies to Isis.

Taken together with the congressional leadership’s shrug, Obama has stripped the veneer off a contemporary fact of American national security: presidents make war on their own, and congresses acquiesce.

The constitution envisions the exact opposite circumstance. A 1973 reform, the War Powers Resolution, attempted a constitutional restoration in the wake of the Vietnam war, ensuring that the legal authorisation for conflict deployments were voided after 60 days. Yet its restrictions on military action have proven far less durable in conflicts like Grenada, Kosovo, Libya and now the 2014-vintage Iraq war.

For the Obama administration, an allergy to congressional authorisation is enmeshed with the president’s stated desire to end what he last year termed a “perpetual war” footing. It has led Obama in directions legal scholars consider highly questionable.

Some of Obama’s legislative brushoffs are straightforward. The administration did not seek legislative authority for its 2011 Libya air war, something Congress was unlikely to grant. Scepticism also mounted in Congress last year when Obama proposed attacking Syrian dictator Bashar al-Assad. Secretary ofstate John Kerry told the Huffington Post that Obama could bomb Assad even if Congress voted against it.

But not only has Obama rejected restrictions of his warmaking power, he has also rejected legislative expansions of it - a more curious choice.

In 2010, shortly after the Republican takeover of the House of Representatives, the incoming chairman of the armed services committee, Buck McKeon of California, endorsed passing a new congressional authorisation for the so-called war on terrorism. McKeon reasoned that a mutating terror threat had pushed the legal boundaries of the brief 2001 AUMF and that a new generation of legislators had not granted their endorsement.

Yet when McKeon’s committee invited the administration’s thoughts, its representative rejected the effort. Jeh Johnson, then the Pentagon’s chief lawyer and now secretary of homeland security, said the 2001 law - passed before al-Qaida’s contemporary affiliates in Yemen, Somalia and north Africa existed, let alone the emergence of Isis, which is no longer part of al-Qaida - provided “sufficient” legal authority for contemporary US counterterrorism.

According to several administration officials over the years, Obama has been wary that Congress will offer up new laws that entrench and expand an amorphous war that, in his mind, he has waged with the minimum necessary amount of force. Obama last year advocated the eventual repeal of the 2001 authorisation - as well as the 2002 congressional approval of the Iraq war - to aid in turning a page on a long era of US warfare.

Yet on Wednesday a senior administration official told reporters that the 2001 authorisation covered the war against Isis. Legal scholars have already debated its coverage of al-Qaida affiliates that did not exist in 2001. Isis, however, is not an al-Qaida affiliate, having been specifically disavowed by al-Qaida’s leader, Ayman Zawahiri. Ken Gude of the liberal Center for American Progress, a thinktank close to the administration, tweeted that he was “utterly shocked” the administration would contend the 2001 authority applied - an argument he had earlier in the day called “laughable.”

Asked to explain the administration’s reasoning, a different senior US official acknowledged the “split” between al-Qaida and Isis but indicated the administration considered it legally immaterial. In an email, using the administration’s preferred acronym for Isis, the official wrote:

Based on ISIL’s longstanding relationship with al-Qa’ida (AQ) and Usama bin Laden; its long history of conducting, and continued desire to conduct, attacks against U.S. persons and interests, the extensive history of U.S. combat operations against Isil dating back to the time the group first affiliated with AQ in 2004; and Isil’s position - supported by some individual members and factions of AQ-aligned groups - that it is the true inheritor of Usama bin Laden’s legacy, the President may rely on the 2001 AUMF as statutory authority for the use of force against Isil, notwithstanding the recent public split between AQ’s senior leadership and Isil.

Obama’s read on Congress has merit. Legislators who endorse congressional authorisation of war against Isis have offered packages that already look beyond the group. Representative Frank Wolf, a Virginia Republican, would give Obama and his successors power to attack all groups sharing “a common violent extremist ideology” - not defined - with Isis and contemporary al-Qaida affiliates. A bill from Senator James Inhofe, an Oklahoma Republican, would empower the president to confront Isis “and any successor terrorist organisation.”

However, the prevailing view in Congress driving a deferral of legislative authorisation for the Isis war is political. Neither Republicans nor Democrats wish to introduce a wild card into the forthcoming congressional elections. Representative Jack Kingston, a Georgia Republican who favours a vote, observed to the New York Times that many of his colleagues reason: “We can denounce it if it goes bad, and praise it if it goes well and ask what took him so long.” Explanations like those contextualise Congress’s diminishing dissatisfaction with violations of the War Powers Resolution over the course of four decades.

Still, that confluence of interests between Obama and the legislature has left Congress on the margins of what might be considered the Third Iraq War. Members of the US public who do not want a return to war in Iraq, nor an expansion of war into Syria, are left without a mechanism to prevent it.

While Obama may think of himself as a bulwark against perpetual US war - and while his political adversaries consider him insufficiently martial - his actions tell a different story. Obama’s foreign-policy legacy is marked by escalating and then extending the Afghanistan war beyond his presidency; empowering the CIA and special-operations forces to strike on undeclared battlefields in Pakistan, Yemen, Somalia, and Libya; the 2011 Libya war; and now returning US warplanes to the skies above Iraq, and, soon, expanding their mission to eastern Syria.

Though Obama typically forswears conventional ground combat in his wars, a factor that tends to blunt congressional outrage, the new war Obama unveiled on Wednesday looks like a different test case. His ostensible prohibition on US ground “combat” forces in Iraq elides the 1,100 ground troops he has ordered back into Iraq since June, a figure certain to expand once the US military revitalises training for its Iraqi counterparts and Syrian anti-Isis rebels. Administration officials anticipate a years-long war against the well-financed Isis, and any vote Congress will cast will come after it has begun, making legislative rejection unlikely. All that creates a precedent for future presidents: shoot first, ask permission later, if at all.

The American and global publics can reasonably ask what 13 years of US war have durably achieved. One answer, unlikely to have been anticipated by the architects, caretakers and practitioners of this conflict, is the hobbling of legislative restrictions on war enshrined in the constitution, and the expansion of a legal authority Obama said last year kept the country on an unacceptable footing of perpetual war.