Trump wants a US 'Iron Dome' the military hasn't asked for

Former President Donald Trump is reviving his promise to build "the best Iron Dome in the world," a version of the air defense system built by Israel that experts say probably wouldn't make much sense for a country with friendly neighbors like Canada and Mexico.

In a conversation Monday night with Elon Musk on his social media platform X, Trump defended his idea by saying "it just takes one maniac to, you know, start something."

"Israel has it. We're gonna have the best Iron Dome in the world. We need it, and we're gonna make it all," he told Musk.

Here's a look at what Iron Dome is, why experts question its utility for the U.S. and how Trump might actually be talking about something else:

What is an 'Iron Dome'?

Iron Dome was developed by Israel's Rafael Advanced Defense Systems to knock down short-range rockets, such as the ones lobbed from the border of Lebanon by the Iran-backed group Hezbollah.

Together with other systems like David's Sling, which is designed to intercept ballistic and cruise missiles, Israel's Iron Dome is part of a nationwide missile-defense shield over the country, whose residents live under the near-constant threat of attack from its neighbors.

The multi-billion dollar system was credited with helping to knock down hundreds of missiles and drones last April when Iran launched an attack. Israel's Iron Dome could play a major role in coming days if Iran follows through on its threat to attack Israel again, this time in retaliation for the recent killing of a Hamas leader visiting Tehran.

As part of a broader policy to help defend Israel, the U.S. has paid some $3 billion to Israel to date for Iron Dome batteries, interceptors and maintenance costs, according to a recent analysis by the Congressional Research Service.

Could the US benefit from its own 'Iron Dome'?

The short answer from experts is probably not. With allies north and south of the U.S., and oceans on either side, the U.S. doesn't face the same kind of short-range missile threat as Israel.

That reality probably explains why the military hasn't asked Congress to pay to build one. A U.S. defense official told ABC News last month that U.S. Northern Command -- -- the military combatant command charged with defending the homeland from foreign missiles -- has not expressed interest in a larger, nationwide missile defense system either.

The military already employs multiple systems "that together provide agility in responding to potential threats, which increases available options for the nation's leaders," the official, who spoke on condition of anonymity, said in July.

Among those systems already in place in the U.S. is the Ground-Based Midcourse Defense program, which was designed to knock down rogue long-range missiles from a country like North Korea. It's possible the U.S. could try to bolster the capability of this existing system to wipe out any threat of a large-scale attack from a country with a hefty arsenal like Russia.

But expanding that system to cover every inch of the U.S. though would likely cost billions of dollars at a time when the country also is trying to protect against attacks in cyber and space. China and Russia are now pursing hypersonic weapons, while administration officials this spring acknowledged Russia's pursuit of nuclear capabilities in space, greatly complicating what it means for any one system to keep the U.S. safe.

So why does Trump keep talking about building an Iron Dome?

Trump often latches on to the idea of building things for the country because it's a tangible example of a policy idea.

In the case of 2016, Trump talked about building a border wall as a way of saying he would take illegal immigration seriously, even as experts warned the idea wasn't practical. By the time Trump left office, only about a quarter of the U.S. border with Mexico had new fencing -- most of which replaced smaller existing structures.

In the case of an Iron Dome, Trump seems to be signaling another shift in U.S. policy -- away from diplomatic engagements with other countries in favor of isolationist policies that rely on heavy military spending to deter adversaries.

For its part, the GOP endorsed the idea as part of its official policy platform at the Republican convention in July.

Elbridge Colby, who served as deputy assistant secretary of defense for strategy and force development during the Trump administration, said last July that he saw the new GOP platform as an intentional shift.

The GOP focuses on protecting the homeland, "not looking for monsters to destroy," Colby said.