Gen. Joseph Dunford is expected to issue a note to all military forces as soon as this week.
Dunford will "remind the troops of the need to remain apolitical in order to sustain Americans' trust and confidence in them," said the chairman's spokesman, Capt. Gregory Hicks.
The U.S. military has a longstanding tradition of remaining out of politics so that commanders are free to give any president their best military advice without it being seen as part of a political agenda.
Hicks noted that the issue often comes up during the presidential campaign season. Troops are reminded not to appear in uniform at political events, for example.
But this year, the issue has grown more public -- and Dunford's letter is aimed at senior commanders as well as the troops -- because of the national security debate on the campaign trail.
Some of the candidates' ideas, such as Donald Trump's statements about waterboarding terrorists
and Sen. Ted Cruz's proposals for "carpet bombing" ISIS,
have sparked intense debate and led to top generals receiving questions on the policies when they appear at news conferences and testify on Capitol Hill.
Dunford "sees a trend line, he wants to address it," Hicks said. "Our ethos of an apolitical nature is the foundation of the trust the civilian leadership has in the unfettered and unfiltered advice and counsel of the military."
Dunford was recently asked if NATO was obsolete, a position advocated by Trump
. Other generals have been asked about carpetbombing and waterboarding.
In some recent instances, top generals have answered questions by the media apparently unaware they were based on statements from candidates.
The Pentagon, like the Intelligence community, is now in the early stages of planning for a presidential transition.
A small number of defense officials have been assigned to begin assessing what must be done to prepare for a transition, including what types of briefing materials and intelligence information will be needed.
The military and Intelligence community will begin to brief both the Republican and Democratic nominees after the political conventions. But fully detailed briefings with the highest level of classified information will not happen until after Election Day.
Dunford, as well as several other top military leaders, will still be in their jobs when the new president takes office in January 2017, so the chairman thinks it's imperative that the individual not see any political leanings among those military officials, defense officials said in explaining Dunford's concerns.