Kennedy’s great expansion was in the area of foreign policy, from soft programs like the Peace Corps to more aggressive options, like the Bay of Pigs and the Vietnam War. The latter, which his successor Lyndon Johnson expanded, is an infamous example of executive power: The war killed more than 58,000 Americans over more than a decade. Though the war was a cataclysm that soured the U.S. on foreign wars for some time, presidents haven’t surrendered the power to deploy the military overseas. At the moment, U.S. forces are involved in wars in Iraq, Afghanistan, and Syria and have extensive presence in parts of Africa, in Yemen, and elsewhere, even though Congress hasn’t declared a war since 1942.
Following 9/11, Congress passed two Authorizations for Use of Military Force, against al-Qaeda in 2001 and for the full-scale invasion and occupation of Iraq in 2002. Obama later launched bombing raids against Libya, saying the effort did not require congressional authorization. He also used the original 2001 AUMF to justify attacks on isis, a group that, as critics noted, didn’t even exist when Congress passed the AUMF.* But Obama blinked when it came to Syria, deciding that he couldn’t launch airstrikes on the Assad regime without Congress’s say-so.
Bush and Obama both pushed the limits of presidential authority and the Constitution on civil liberties on national-security grounds. The Bush administration was repeatedly scolded by courts for its handling of detainees from the war on terror. The government asserted the authority to torture prisoners, in part by refusing to characterize its interrogation techniques that way. Bush also established sweeping new surveillance powers.
Obama took office promising to curtail Bush’s overreach, but his record is a mixed bag. Obama banned and investigated torture but declined to punish anyone for it. He continued massive surveillance, as revealed by Edward Snowden. Obama also asserted the right to kill American citizens overseas without a trial, a sweeping power, though used rarely.
What’s surprising is how limited, so far, Trump has been in expanding executive power where other presidents have. Despite his promise to not engage in foreign wars, Trump ordered two separate missile attacks on Syria. The OLC told Trump that such strikes were not a war but were in the national interest, but this justification follows what Obama’s OLC told him.
When the government undertakes increased surveillance, it tends not to say so, so it’s possible that Trump is expanding presidential power in that area. There are reasons to doubt that, though. Trump has had the rockiest relationship with the intelligence community of any president, perhaps ever, and has clashed with these agencies in particular over the Russia investigation. Because of his many, and so far all debunked, claims of having been improperly surveilled during the campaign, he has evinced a personal skepticism of intelligence collection and even briefly came out strongly against an intelligence bill his administration backed, before apparently being talked down by aides.