On Trump's Declaring a National Emergency over the Border wall Posted on 1/5/2019 at 16:57:44 by Bill Crawford, (14)
With Trump's stating that he might declare the border wall a national emergency matter in order for him to move forward with funding without the approval of Congress, I checked out the below-linked article, which indicates that the Supreme Court overruled Truman when he attempted to have the government keep the steel mills running during the Korean War, in order to stop a pending strike, citing a national emergency. Among the reasons given by the court for the overrule was that the situation was not severe enough to justify the President's taking such action. However, the court implied that under more extreme circumstances, such an action by a president might be constitutional.
I think the Court would take similar action in this case, with the Chief Justice casting the deciding vote.
[With regard to the danger to our national security due to crimes being committed by illegal aliens, the below article indicates that that's not the case. (https://www.npr.org/2018/06/22/622540331/fact-check-trump-illegal-immigration-and-crime)].