India, Feb. 8 -- President Donald Trump has taken the unusual step of tapping military leaders for high-level diplomacy, sending the top US commander in the Middle East to talks over Iran's nuclear program and positioning the Army secretary as a key negotiator on ending the Russia-Ukraine war.

Admiral Brad Cooper, head of US Central Command, for the first time joined indirect US-Iran talks Friday in Oman, appearing in his dress uniform as a reminder of the American buildup of military might in the region.

As Army Secretary Dan Driscoll reprised his role at Russia-Ukraine talks this week, he worked to keep the conversation going with Ukrainian officials in the downtime between sessions, according to a person familiar with the negotiation...