Wait a minute, Congress has never passed any legislation that says no wall, ergo he isn't going against the will of Congress. If Congress doesn't do that, then the courts have in the past considered such action as tacit approval, given that they gave him the power to declare an emergency with no limitations. And in fact, Congress just passed legislation that authorized the wall, plus they authorized it twice before in the past.
Congress has certainly allocated money for specific purposes such as military construction and drug interdiction programs which Trump plans to use. If the president uses funds approved by congress for these specific programs to build a wall, he certainly is going against the will of congress. Congress does not have to pass a law forbidding the use of billions of dollars to build a new barrier wall, or any other major project because funds spent by the president on major projects must be approved by congress. The president's legal authority to spend money as he pleases is very limited.
If Trump declares a border emergency, Section 2808 of Title 10 of U.S. law provides authority to build a wall: When a national emergency “requires use of the armed forces,” the Defense Department “may undertake military construction projects . . . not otherwise authorized by law that are necessary to support such use of the armed forces.”
Section 2808 of the Emergencies Act of 1976 allows the secretary of defense to “undertake military construction projects” that are “not otherwise authorized by law that are necessary to support such use of the armed forces” in the event that the president declares a national emergency. A parallel statute, <33 U.S. Code § 2293, allows the secretary, during such a declared emergency, to redirect “the resources of the Department of the Army’s civil works program, including funds, personnel, and equipment, to construct or assist in the construction, operation, maintenance, and repair of authorized civil works, military construction, and civil defense projects that are essential to the national defense.”
There are no parameters governing the criteria for such a declaration or defining the types of projects that might be in order. The only thing the secretary of defense has to do is to communicate the nature of the decision and its costs to the relevant congressional committee, but he does not need to obtain congressional approval. One might legitimately feel uncomfortable with such broad authority delegated to the president, but nonetheless, this is the law on the books. Congress has delegated a lot of authority to the president for the purpose of protecting our sovereignty and security.
Given that Congress has delegated such authority to spend money under the NEA, I have no problem using it on a project that has already been authorized under statute. Section 102(a) of the 1996 The Illegal Immigration Reform and Immigrant Responsibility Act provides that the secretary of homeland security “shall take such actions as may be necessary to install additional physical barriers and roads … in the vicinity of the United States border to deter illegal crossings in areas of high illegal entry into the United States.” The Secure Fence Act of 2006 mandated that DHS “construct fencing “along not less than 700 miles of the southwest border.”
Long story short, Congress has in the last several decades delegated much of it's authority to the President, and now it's biting them in the ass. It's a hard sell IMHO to claim that Trump is subverting Congress or going against its will when in the past they have passed legislation to do what he's doing now.