President Trump was clear about why he fired national security adviser John Bolton: "I disagreed strongly with many of his suggestions, as did others in the administration, and therefore I asked John for his resignation."

Trump, of course, has the right to a national security adviser who reflects his views. But he'll be making a serious mistake if he picks a sycophant to replace Bolton. What Trump needs is an adviser whom he trusts, someone who can effectively manage the national security interagency process. The job of a national security adviser is not to advocate for the president's ego, but to articulate a range of national security policy options and then advocate for the one he or she believes is best.

While Bolton and Trump regularly disagreed on what policy to pursue, perhaps hopelessly so, the president should hold true to the benefits of hearing an alternative viewpoint.

Regardless, Trump must rapidly appoint a credible successor. Recent global events have proven that Trump's national security policy is losing cohesion.

First, we've seen the failure to seize the Iranian oil tanker Adrian Darya 1 as it delivered cargo to Syria in an overt breach of its pledges not to do so. The Justice Department had issued a warrant to seize the vessel, but no U.S. action was taken. This is almost certainly because Trump refused to authorize the U.S. Navy to do the job. But the consequence is clear: Even as his instincts on Iran rightly balance those of Bolton and Secretary of State Mike Pompeo, Trump's maximum pressure strategy toward Iran is starting to crumble. In Adrian Darya 1, the Iranian hardliners now have an explicit example of Trump's hesitation.

We've also seen escalated North Korean ballistic missile tests in recent months. Trump suggests these tests are not problematic, but he is wrong. They reflect Kim Jong Un's increasing willingness to test the limits of Trump's patience in the ongoing nuclear standoff.

Toward Russia, also, Trump's policy seems increasingly uncertain. The president has returned to his silly suggestion that Russian President Vladimir Putin be readmitted to the G-7. Trump has also delayed the delivery of weapons to Ukraine. These decisions reward Putin for his continued aggression and degrade the American-led democratic international order.

As he looks to appoint a new adviser, Trump needs to find someone who can restore clarity of purpose to his national security policy.