One way is to reallocate funds previously approved. It has been done many times before with other Administrations.
Only Congress can do that, too.
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How then can the president take the nation too war if necessary without going thru the Congress as he is able too do via the War Powers Act?
Suppose the President deems the wall a national security issue? Would he then be able to allocate funds off budget?
It is my understanding that he can only do so for 60 days without Congressional approval. After that, Congress needs to approve funding for it.
Same with the wall. All funding comes from congress. They hold the purse strings.