WASHINGTON — The Trump administration, which came into office pledging to end “endless wars,” has now embraced weapons prohibited by more than 160 countries, and is readying them for future use. Cluster bombs and antipersonnel land mines, deadly explosives known for maiming and killing civilians long after the fighting ended, have become integral to the Pentagon’s future war plans — but with little public rationale offered for where and why they would be used.

These new policies, endorsed by Defense Secretary Mark T. Esper, can be traced to 2017 when the Pentagon chief at the time, Jim Mattis, was drafting a military strategy that named Russia and China as the United States’ great power rivals. Both have significant ground forces, and mines historically have been used to deny an adversary’s troops the ability to advance on the battlefield.

In a news conference on Monday, the Pentagon’s chief spokesman, Jonathan Hoffman, said that the policy change “was the result of an extensive conversation” with different departments of the executive branch. It is intended “to provide the commanders on the ground nonpersistent munitions that are necessary for mission success in major contingencies in extreme or exceptional circumstances,” he said.

Mr. Hoffman declined to specify who had requested the policy change.

Former Defense Department officials said the debate about reintroducing land mines and other so-called area-denial weapons came to a head in 2017 as the administration analyzed Russia’s rapid invasion and annexation of Crimea.