Actually, U.S. presidents have been going to war without Congress since the beginning

In the modern era, it's become increasingly common for presidents to send troops into battle without authorization from Congress -- a practice many argue is unconstitutional. During the 2011 intervention in Libya for intance, Yale law professor Bruce Ackerman wrote that "Barack Obama's administration is breaking new ground in its construction of an imperial presidency -- an executive who increasingly acts independently of Congress at home and abroad." It may be unconstitutional, but how unprecedented is it?

There's a general consensus that the imperial presidency model of warfighting began with Theodore Roosevelt and expanded dramatically after World War II -- the last time Congress formally declared war. The War Powers Resolution of 1973 was meant to check the president's ability to do this, but several administration's have skirted it. A paper by William D. Adler in Presidential Studies Quarterly analyzes the "small wars" of early U.S. history and found that the tradition of the president acting as "generalissimo of the nation" as the pseudonymous "Cato" put it in the Anti-Federalist Papers, goes back much further than we think.  

America's two largest pre-civil war conflicts -- the War of 1812 and the Mexican American War -- were both organized by Congress, though James Polk played a major hand in instigating the second one. But there were dozens of other conflicts, sometimes involving thousands of combatants. These included at least 10 major conflicts with Indian tribes between the 1790s and 1850s as well as " the Quasi-War with France during the late 1790s; the conflict with the Barbary pirates during the 1800s (which had been an ongoing problem since Washington's presidency); General Andrew Jackson's repeated invasions of Spanish Florida; naval skirmishes in Sumatra during the 1830s; the so-called Patriot War from 1839 through 1841 on the northern border; "bleeding Kansas" during the 1850s; and the Mormon war in Utah later that decade."

Adler argues that in all these conflicts, Congress -- if involved at all -- generally simply delegated warmaking powers to the executive branch and only excercised oversight after the fact. This was particularly this case in conflicts with Indian tribes, which were generally overseen by the War Department in cooperation with state governments without Congressional deliberations. 

In a scenario familiar from the Iraq War era, Congress often didn't realize quite how much power it was granting to the president. Adler recounts what happened in 1792 when Congress agreed to George Washington's request for 5,000 additional troops to fight Indian tribes in the Ohio Territory: 

During the conflict, Congress does not seem to have comprehended just how much power
it had delegated to the executive branch.  As Richard Kohn explains, when St. Clair asked for permission to call up the militia, Congress authorized it, but "only later, after Washington cited the amendment as authorization for Harmar's campaign, did Congress understand fully that it had given the President permission to wage war on his own authority." Similarly, when Congress appropriated $1 million to fund the ongoing operations, this was taken by the administration as an implicit authorization to continue the fighting; at no time did anyone think  of asking for a formal resolution allowing the war to progress.

Early American presidents also had a habit of going to war first and asking permission later, such as when James Madison authorized the Army to seize territory in West Florida in 1810. As a general, Andrew Jackson invaded Florida three different times without asking permission from the president -- not to mention Congress. As president, Jackson continued to follow the imperial model, interpreting the Indian Removal Act of 1830 as carte blanche to use military force against Indian tribes who refused to relocate, an event now known as the Trail of Tears. This included the forced removal of the Florida Seminoles, which the largest U.S. military engagement since the War of 1812, with nearly 9,000 soldiers involved.

In addition to the question of presidential powers, the reconsideration of early America's small wars is also interesting in light of the question of just how many wars America is currently fighting, which I discussed a bit last week. The persistent state of ongoing, low-level military conflict we have now may not actually be that much of a historical aberration for the U.S., though the fact that there may be a slight legal and political resemblence to early America's Indian Wars is hardly reassuring about the current state of affairs.