With one of his new executive orders on Monday, President Trump called for the construction of an Iron Dome-like defense shield over the United States.

It may sound like a good idea to anyone who has watched footage over the years of Israeli interceptors blasting apart rockets and missiles fired into that country by regional foes. But in practical terms, Israel is the size of New Jersey, and the missiles fired into Israel are often unguided, slow-moving projectiles lobbed from nearby — not the world-spanning missiles U.S. military planners fear most.

It’s cat-and-mouse, but simply put: It’s cheaper, quicker and easier to build missiles and offensive systems than it is to intricately engineer a shield to defeat them. It’s difficult work to develop the radars, sensors, interceptors and associated systems that detect, track and engage flying missiles. It’s even more complex to hit a ballistic missile as it zips more than 14,000 miles per hour. The metaphor most often used in military circles when it comes to shooting down an intercontinental ballistic missile with an interceptor is “hitting a bullet with a bullet.”