But even if you can credibly argue that a national emergency exists, and that the emergency may actually require the use of military assets, how do Trump’s defenders argue that the construction of his wall is actually already “authorized”? Easy, they say. Congress passed the Secure Fence Act in 2006, and it authorizes the construction of fencing on defined sections southern border.

But wait. That project is almost entirely complete. The original act required specific fencing, but when DHS complained that different parts of the border required different kinds of barriers, Congress amended the act to merely require DHS to erect fencing, physical barriers, or “roads, lighting, cameras, and sensors” in specific areas along the Mexican border. By 2011, DHS declared that it had finished 649 out of the planned 652 miles of fences and other barriers. In other words, the construction project — as defined by the statute — was more than 99 percent complete.

So, is the argument that Trump can declare a national emergency to upgrade a completed fencing project? But where is the authority for the upgrade? In our system of government, Congress holds the power of the purse, and it authorizes new construction — even where old construction exists.