A redeployment of Russian troops to Ukraine from Syria is raising hopes in Tehran for increased presence on Israel’s northern border — but how eager is Moscow to let the Islamic Republic dominate a crucial Russian stronghold in the Mideast? 

According to some reports, as many as 20,000 Russian troops and mercenaries affiliated with Moscow who had long been deployed in Syria and Libya have been recently dispatched to the Donbas region and other areas in Ukraine.

A Saudi-backed news channel, Al Arabiya, documented a Russian handover of Syrian bases to Iran. The report was accompanied by a video showing what the channel reported were 25 troop-carrying trucks leaving for an airport, from which the destination presumably was Ukraine.

Just how significant is the Russian abandonment of Syrian bases? Reliable figures are difficult to document in the war-torn country. Yet evidence grows that each spot vacated by Russian troops is taken over by Iran’s Islamic Revolutionary Guard Corps, Lebanese-based Hezbollah, or local Tehran-backed militias. 

Iran will attack Israel from “wherever it feels necessary,” the IRGC Quds Force commander, Esmail Qaani, said last week. 

A small number of American troops and intelligence personnel are stationed in eastern Syria, where they cooperate with a Kurdish faction in the war against ISIS. On April 7, four American soldiers were injured after an Iranian-backed militia attacked their base there. 

Yet, America has no influence over the rest of Syria. President Obama’s failure to enforce his own red line over the Syrian strongman’s use of chemical weapons was seen by the Kremlin as a green light to send troops and mercenaries to aid President Assad’s side in the civil war. 

President Putin became Mr. Assad’s savior and thus re-established Russia’s dominance over Damascus. In the process, he perfected in Syria some of the Russian atrocities now being documented in Ukraine. 

Currently Moscow and Tehran — both operating under American and global sanctions — are tightening their alliance. Russian diplomats serve as an intermediary between Washington and Tehran in the negotiations over the 2015 nuclear deal’s renewal. Most recently these diplomats demanded that Russian trade with Iran be exempted from Ukraine-related sanctions.

Although Iran and Russia have cooperated for a decade in shoring up Mr. Assad’s hold on power, they have their differences, including years-long competition over areas of influence in Syria. 

Moscow’s interests include its control over two major Syrian ports, Latakia and Tartus, which gives the Russian navy access to warm water and a foothold in the Mediterranean, which it considers crucial in its global competition with America. 

“Syria is still very important for Russia, and Moscow wouldn’t want Iran to erode its influence there,” the founder of Alma, a research center specializing in Israel’s northern borders, Sarit Zehavi, says. 

A former lieutenant colonel in the Israeli Defense Force, Ms. Zehavi says her group, which plans shortly to publish a detailed paper on the Russian-Iranian dynamic in Syria, is yet to document a “major” Russian troop movement out of the country.  

Since Russia intensified its presence in Syria in the middle of the last decade, Jerusalem and Moscow have maintained good relations that allowed the Israeli air force to operate almost freely over Syrian skies. The Israelis conduct periodic attacks on IRGC and Hezbollah bases in Syria, and intercept major Iranian arms deliveries through the country to Hezbollah in Lebanon.   

The officially undeclared ties between Moscow and Jerusalem have cooled off recently, after Israel voted against Russia at the United Nations. Further, the Israeli foreign minister, Yair Lapid, accused Moscow of committing war crimes in Ukraine. Moscow retorted with harsher-than-usual condemnation of Israel’s treatment of Palestinians.

More worrisome for Israel, the state-owned Russian press reported details of a recent IDF attack in Syria. The disclosures were widely reported in the Israeli press, portrayed as a possible sign that Moscow intends to cool off relations. 

Yet that air attack — targeting a Syrian base where Iran and Hezbollah upgrade the accuracy of missiles — took place last week. Unusually, it was conducted in daylight and Russian planes, which dominate the Syrian airspace, did not interfere.

According to Ms. Zehavi, the Russian press accounts of the attack were mostly based not on secret intelligence but on recycled material. 

Moscow benefits from the tacit agreements with Israel over Syria, Ms. Zehavi said, adding, “I assume that there is an intense debate inside the Kremlin,” where some advocate a toughened anti-Israel line while others advise caution and warn against allowing Iran to control too much Syrian territory.

As the Russians get increasingly involved in Ukraine, Syria is one of many other global spots where alliances could shift quickly. Syria is worth watching intently — not only by Israelis, but by Americans.