After more than 30 years of research and more than $200 billion, the nation’s ballistic missile defense program remains riddled with flaws, even as the threat from North Korean missiles escalates.

Still, President Trump persuaded Congress to increase spending for the program to $14 billion, from $10 billion, in the 2018 budget, claiming in his national security strategy that plans to push the system “will include the ability to defeat missile threats prior to launch.”

Mr. Trump is overselling the program, an interlocking network of interceptors, radars, sensors and so-called kill vehicles. He has boasted that the system is 97 percent effective in preventing limited-scale attacks; the truth is more like 50 percent. So its defense against North Korean weapons is hardly a sure thing.

The program’s failings were on display as recently as last month, when, during a test off the Hawaiian coast, an interceptor missile launched from a test site in Kauai failed to hit its target, an incoming dummy missile. That was the second failure in three tests of the interceptor, the SM-3, which is intended to be a mainstay of American regional missile defense systems being deployed in Romania, Poland and Japan to guard against medium-range missiles. The Pentagon has not disclosed what went wrong.