I'm fairly certain no sitting US President has ever led troops in battle (though many have led troops prior to holding the oficce) but George Washington did personally lead an army to crush the Whiskey Rebellion in 1794, though the rebellion dispersed before any shots were fired. The rebellion was essentially over the federal governments ability to tax commodities. The rebels argued that they did not have adequate representation for the government to tax them and obviously the Washington administration disagreed.
As far as modern heads of state leading men in combat I can't point to anything secifically. I'm sure there is an example somewhere out their though probably not for an industrialized nation.
Alternate question, have any national leaders personally engaged in battle since 1900?
Certainly, although not during their presidencies, if that's the question you're asking. Off the top of my head, the following presidents (narrowing that from "national leaders") who have served since 1900 had combat experience:
Lyndon Johnson served in the Navy in World War II, but in a non-combat role (he did ride along in a bomber on one highly disputed combat mission). Richard Nixon also served in the Navy, but as a Quaker, he was put into a non-combatant role. Jimmy Carter was in the Navy from 1946-53, but did not see combat. Ronald Reagan was in the USAAF during WWII, but did not see combat.
Winston Churchill quit being a cabinet minister during World War I and then joined the army and fought in the war.