Mr. Obama’s defenders may point to the precedent set by Bill Clinton, who continued the NATO-led bombing of Kosovo in 1999 beyond the 60-day limit without congressional assent. But Mr. Clinton halted the campaign before the 30-day withdrawal period ended.

In 2011, when Mr. Obama continued to bomb Libya after the 60-day limit, his lawyers argued that America’s supporting role in the NATO campaign was not substantial enough to qualify as “hostilities” under the 1973 resolution. This claim provoked howls in Congress and the legal community, but the death of the Libyan dictator, Col. Muammar el-Qaddafi, took the steam out of the debate before it could be resolved.

Even if the Obama line on Libya were accepted, however, it fails to justify his current move. Rather than “leading from behind” by backing NATO, Mr. Obama is now taking the lead in an open-ended campaign, extending from Iraq into Syria, that could last years. If this isn’t commencing “hostilities,” what is?

Senators and representatives aren’t eager to step up to the plate in October when, however they decide, their votes will alienate some constituents in November’s midterm elections. They would prefer to let the president plunge ahead and blame him later if things go wrong. But this is precisely why the War Powers Resolution sets up its 60-day deadline: It rightly insists that unless Congress is willing to stand up and be counted, the war is not worth fighting in the name of the American people.

If Mr. Obama changes course, as he did last September, and submits to the commands of the War Powers Resolution, Congress can demonstrate that, despite all its dysfunction, it can still rise to the occasion. There are hawks and doves on both sides of the aisle, and leaders of both parties have signaled a willingness to engage in a serious debate.

But for now the president seems grimly determined to practice what Mr. Bush’s lawyers only preached. He is acting on the proposition that the president, in his capacity as commander in chief, has unilateral authority to declare war.

In taking this step, Mr. Obama is not only betraying the electoral majorities who twice voted him into office on his promise to end Bush-era abuses of executive authority. He is also betraying the Constitution he swore to uphold.