Campaigning in 2008 as a critic of the expansion of US wars abroad under the post 9/11 Authorization for the Use of Military Force (AUMF), President Obama ended up spending his entire eight years in office falling back on the AUMF as his go-to pretext for all the wars he started.
Despite the AUMF explicitly centering on al-Qaeda and others involved in 9/11, President Obama insisted it was fine to use it to attack the Gadhafi government in Libya, and earlier this year to attack Somalia’s al-Shabaab, which didn’t even exist when 9/11 happened.
While there has been some bipartisan criticism in Congress of this, it’s never been nearly enough to stop a war, and were more than willing to ignore Obama not seeking a new authorization for a war. The courts likewise have refused to get involved. This has only underscored Bush Administration policy, meaning that Obama could unilaterally declare presidential wars with no oversight.
And by extension Donald Trump. The president-elect takes power in mid-January, and will inherit a de facto precedent that he can start basically any war he wants to, and can tie it to the 2001 AUMF, even if it makes no sense, safe in the knowledge that no one else in power will call him on it.
Trump hasn’t indicated that he intends to use this power, and has repeatedly criticized the US for being in too many wars as it is, talking about abandoning nation-building and limiting the number of wars. Of course, Obama was also critical of the wars he inherited at first, and President Bush similarly promised a more humble US foreign policy in 2000.
But even if Trump doesn’t follow the trend of becoming much more hawkish once he’s inaugurated, there’s no sign yet that he intends to reverse the precedents set by his predecessors, meaning that presidents to come will continue to have this power, barring a serious challenge.