Donald Trump is looking forward to being president again:
An exultant Donald Trump has promised to act with “historic speed and strength” when he returns to the White House on Monday.... “Starting tomorrow, I will act with historic speed of strength and fix every single crisis facing our country,” he said.
....Trump said he would instruct the military to construct an “iron dome” missile defence system.
That sounds pretty good. But I have a question: Does Trump even know what Iron Dome is, aside from having a cool name?
The short answer is that it's an Israeli system for shooting down short-range rockets. The United States (a) already has layered missile defense systems (Patriot, THAAD, C-RAM, NASAMS, Aegis) that do much the same thing, (b) is already developing a long-term, more integrated missile defense system called IFPC, and (c) doesn't use any of these things to protect the US itself because, unlike Israel, we're under no threat from short-range missiles.¹
An "Iron Dome" for the US would be a system that protects against long-range ICBMs. As even Trump should know, he doesn't need to order anyone to construct this because we've been working on it since the Reagan era.² It just doesn't work yet. Nevertheless, we spend about $10 billion per year just on the Missile Defense Agency, which accounts for roughly half of all missile defense spending:

I suppose eventually somebody will tell him this. Or else they'll just salute and then keep doing whatever they're already doing now.
¹There's an exception: we have missile defense batteries that defend the airspace around Washington DC.
²Or well before that if you count Nike-Zeus, Sentinel, and other early ABM systems.