But appointing another retired general to lead the Pentagon will not help return things to normal. Even if a retired general like Mr. Mattis was the right person for the Trump era, that era is over. A legislative exception granted at an exceptional moment should not become the new rule.

After four years of relative, if erratic, autonomy under Mr. Trump, military leaders may chafe when civilian national security leaders ask to check their homework. To some extent, that is healthy. Too much friction can also stop or slow progress, true, but a certain level is necessary for proper governance.

The need for experienced leadership in the Pentagon to manage this friction is vital. As even George Marshall realized, Mr. Biden would be wise to select a strong civilian who is up to the task.