PRESIDENT OBAMA’s declaration of war against the terrorist group known as the Islamic State in Iraq and Syria marks a decisive break in the American constitutional tradition. Nothing attempted by his predecessor, George W. Bush, remotely compares in imperial hubris.

Mr. Bush gained explicit congressional consent for his invasions of Afghanistan and Iraq. In contrast, the Obama administration has not even published a legal opinion attempting to justify the president’s assertion of unilateral war-making authority. This is because no serious opinion can be written…

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.