Russia’s domestic intelligence agency patrols the meandering river, alongside cameras, watchtowers and three rows of barbed-wire fencing.

But Russia itself is almost 200 miles away. And by January, the Russian officers will start leaving.

This is the border between Iran and Armenia, a 30-mile strip that is a pivot point of a head-spinning geopolitical shift. Here in the Caucasus, the mountainous region where Europe meets Asia, Russia and Iran are increasingly seen as rivals, while Western countries are — surprisingly — finding some common cause with Tehran.

This complex, multicountry knot of interests and influences challenges Western conventional wisdom about alliances and could be upended yet again by the re-election of Donald J. Trump in the United States.

In a rare interview last week, Iran’s ambassador in Armenia, Mehdi Sobhani, acknowledged the diverging interests of Russia and Iran in the region, rather than the “strategic partnership” they often profess, banding together against the United States.

“We are not allies,” Mr. Sobhani said. “We have some differences, and we have some mutual interests. It doesn’t mean that we are allied.”