The available public evidence suggests administration witnesses will have a hard time justifying that the mission is in the interest of national security. Here are a few questions that members of the new Democratic majority should pose to DoD witnesses:

What was the requirement to use active-duty forces?

It's not unprecedented for presidents to send military forces to support law enforcement at the southwest border in an election year. Presidents Bush and Obama deployed the National Guard to the border in 2006 (Operation Jump Start) and 2010 (Operation Phalanx). What is different this time? The use of more than 5,000 active-duty forces, in addition to some 2,000 National Guard forces that were already on station. What unique capabilities, not available in the Guard or Reserve forces, were required for this deployment?

The other key difference in this case -- the overtly political purpose of the deployment, as demonstrated by President Trump's regular invocation of it on the campaign trail, where he warned of supposed dangers from a "caravan" of migrants traveling from Central America and claimed a "crisis" on the border. Yet seizures of inadmissible aliens at the southern border are at historic lows, which hardly suggests a crisis is at hand. And, quite apart from the absurdity of using the military for a civilian mission to stop a group of desperate, unarmed migrants, why were most of the forces deployed to south Texas when the caravan arrived in Tijuana?

Who created the false talking points about terrorists?

Administration officials sought to scare the American people by asserting that many terrorists were among those showing up at the southwest border. This lie survived neither scrutiny by the media nor expert analysis, including by the former director of the National Counterterrorism Center, and the Trump team retreated. Who injected these false arguments into administration talking points?