WASHINGTON (AP) — President Donald Trump on Monday asked the Senate to confirm Mark Esper as the successor to former Defense Secretary Jim Mattis, whose resignation last December opened an unprecedented period of senior-level instability at the Pentagon.

The moment the nomination was received by the Senate on Monday afternoon, Esper was required to step out of his role as acting defense secretary — a job he has held since June — until he is confirmed as the permanent secretary. He reverted to his previous position of Army secretary.

Filling in for Esper pending his confirmation is Richard Spen...