President Trump has a long “to do” list, starting with repealing Obamacare, fixing the tax code and building a wall. But there is another kind of a “wall” that demands the president’s attention: The president must make sure the nation has the missile defenses it needs.

Ballistic missiles remain the weapons of choice for our adversaries for several reasons. From anywhere in the world, they can deliver a lethal payload to the continental United States in less than 33 minutes. They are relatively cheap to build, and the technologies used to make them are becoming more widely available.

And they can deliver not just destruction, but doomsday. A nuclear warhead detonated at a high altitude would completely obliterate the electric grid for an enormous area. That could bring modern life to a halt in multiple states, setting survivors back to 18th century.

Being able to defend ourselves from ballistic missile attack is essential. It is also an extremely complicated technological and engineering challenge.

Throughout his two terms in the Oval Office, President Obama left much undone in terms of addressing the ballistic missile threat. He cancelled projects to develop and advance promising missile defense technologies — like the Airborne Laser, an aircraft with chemical laser that successfully shot down ballistic missiles. The Multiple Kill Vehicle, a technology that could make some existing interceptors more effective, was another casualty of the Obama administration’s misguided belief that a strong missile defense would somehow make the country less safe.

Our European allies were left hanging when the administration cancelled efforts to deploy long-range interceptors in Poland and an X-band radar in the Czech Republic. Instead, Mr. Obama began implementing a substitute plan — one that deploys, later than the original plan, less-capable interceptors to Poland and Romania.

The Trump administration should ensure these missile defense deployments remain on track. Further delay would only extend our allies’ vulnerability. But the new administration should also improve the plan by developing a long-range ballistic missile defense interceptor that could be deployed to Europe to augment our ability to protect the homeland.

Currently, the only interceptors that protect the United States from long-range ballistic missiles — like those being developed in North Korea — are deployed in Alaska and California. There are 30 of them.

The plan under President George W. Bush was to strengthen that thin layer by deploying a total of 44 interceptors, possibly even more. But in 2010, Mr. Obama decided we didn’t need that many to get the job done; he decreased the number to 30.

By 2013, the Obama administration recognized that 30 wasn’t enough and decided to go back to the original plan of deploying 44 interceptors. But the dithering has cost our nation several years — years in which we will have to wait before we get better ballistic missile protection.

Mr. Trump will have an opportunity to serve the nation where Mr. Obama failed: spurring the development and deployment of space-based ballistic missile interceptors. Space-based defenses are the most effective way to protect the nation from all types of ballistic missiles — short- and medium-range, as well as long-range.

While still in space, ballistic missiles can’t deploy decoys — a limitation that makes it easier to track and “lock on” to them. Space-based missile defense would allow us to defend against large-scale attacks, not just “a handful” of missiles, which is what the current system is geared to handle.

Would such a system be technologically challenging? Of course. After all, it is a rocket science. Would it be impossible? Not at all. The nation came fairly close with the Global Protection Against Limited Strikes program over 20 years ago. And technologies have advanced exponentially since then, opening the promise of space defenses that are cheaper, smaller and more effective.

Today, the largest obstacle to effective missile defense is not technological but political. Domestically, opponents continue to believe that leaving U.S. cities vulnerable to ballistic missile attacks is somehow “stabilizing.” Internationally, we have been too sensitive to Russian propagandistic objections.

For eight years, these political considerations have prevented us from applying U.S. technology and creativity to shield us from the rising existential threat of ballistic missile attack. The need for an effective missile defense is greater and more pressing than ever.