what is the significance of crossing the Potomac river in the following excerpt? [closed]

Over the last 40 years, Mattis said, there had been some years when relations between the secretary of state and secretary of defense had been so bad they did not speak to each other or even so much as cross the Potomac River and shake hands.

what I guess is that they would cross the Potomac river but not shake hands because of their bad relationship. Can anybody tell me its significance?

The Potomac River splits Washington DC from Arlington VA. The White House is on the east side of the river in Washington while the Pentagon is on the west side in Arlington.

The Secretary of State's office is located in the Truman Building, about a half-mile west of the White House. This is on the east side of the Potomac. The Secretary of Defense's office is located in the Pentagon, on the west side of the Potomac. To see each other face-to-face (and not on Zoom) one of the pair would have to cross the Potomac.

The statement by Mattis is implying that there were times when neither individual felt it was worthwhile to make the journey.