Now, after two and a half years in office, Trump has failed to deliver on that promise. Mexico has not given even a cent for the wall, and assures everyone they will give nothing in the future. And the U.S. Congress has basically done the same -- budgeting only a pittance for the wall.
To date, not a single mile of new wall has been built (although some existing parts of the current wall have been repaired or replaced).
That had Trump worried. He didn't want to have to run for re-election without any portion of new wall being built. That's because any hope he has of being re-elected depends on his followers going to the polls in very large numbers.
So, what does he do? He instructs the Secretary of Defense to use money budgeted for the military to put it towards building the necessary wall. And the sycophant Defense Secretary complied -- shifting $3.6 billion in military construction funds to the wall.
This was not "extra' Money the military had. It was money to be used for badly needed construction. The military said it was crucial they receive that money for construction (barracks, housing, etc.), and Congress believed them and gave them that money. Now that construction won't be done. The government said that construction was only delayed. But that means Congress will have to again budget that money for military construction, and the taxpayers will have to pay more (for a purpose they thought they had already paid for).
These actions by the executive branch are shameful. They should not be able to use money Congress has budgeted for one purpose to do something else. And do it so a lying narcissist can brag to his base about building a wall. But it looks like Trump will get away with this. He will get away with it because the Republicans in Congress care more about keeping Trump happy than protecting the powers of Congress.
Trump needs to be defeated in 2020, and so do all the Republican officials who support him.