From The Economist:

Presidents stamp their priorities on it as they do on any other cabinet department. The first mission listed in the act of Congress creating the department was to “prevent terrorist attacks within the United States”. Paul Rosenzweig, a former deputy assistant secretary for policy at the dhs and now a senior fellow at the r Street Institute, a think-tank, recalls that Mr Bush held “Terrorism Tuesdays”, at which senior officials would brief him on threats. That posed by al-Qaeda receded during Mr Obama’s presidency, leaving him free to focus more on cyber-security and immigration. As Carrie Cordero of the Centre for a New American Security puts it, the dhs really is “an all-hazards department”.

It particularly interests Mr Trump because of its role in immigration enforcement. It has carried out several of his most controversial policies, including building bits of the border wall, separating migrant parents from children, implementing sweeping travel bans and deploying paramilitary forces against protesters in Portland, Oregon and Washington, dc. None of these uses clearly exceeds Mr Trump’s statutory authority; presidents have broad powers and discretion to define and respond to matters of national security.

But, Mr Rosenzweig argues, “this president has taken his discretionary authorities and used them in distorted ways that were never reasonably within the contemplation of those who created those authorities in the first place”. For instance, presidents can deploy bortac, the border agency’s paramilitary troops, to help the Federal Protective Service guard federal property, as Mr Trump did by executive order on June 26th. But none has previously deployed paramilitary forces against the wishes of a state governor—as Mr Trump did in Oregon, where, wearing uniforms without names or identification, they appeared to arrest people far from the buildings they were ostensibly guarding—nor sent them to “sanctuary cities”, from Boston to Los Angeles, in a show of force.