Having successfully propped up the regime of Syrian President Bashar al-Assad against an array of armed rebel groups for more than six years, Iran appears to be preparing the ground for a long-term presence in the war-ravaged country, causing rising alarm in neighboring Israel, its bitter foe, and garnering the attention of Washington.

Iran has spent hundreds of millions of dollars to buttress Syria’s economy, oversees a multinational Shiite militia force to bolster Mr. Assad’s flagging army, and trains Syrian militia networks based on Iran's Basij paramilitary volunteer force.

But Iran’s success and expanding reach into Syria, which serves as Tehran’s vital geopolitical link to its client Hezbollah in Lebanon, has made Syria potentially a key arena if the US wants to undermine Iran’s regional stance, analysts say.

As a consequence, President Trump’s administration has signaled an intention to roll back the Islamic Republic’s influence, not only in Syria but elsewhere in the region.

“The best strategy to roll back Iranian influence is to weaken it in Syria by denying it the resources it has invested in through the Assad regime,” Randa Slim, a scholar with the Washington-based Middle East Institute and an expert on Hezbollah, says without elaborating. “Syria anchors the Iran-Syria-Iraq-Hezbollah axis. Denying Iran that anchor will roll back its influence not only in Lebanon and weaken Hezbollah but in the whole region.”

In Moscow last week, Israeli Prime Minister Benjamin Netanyahu told President Vladimir Putin there could be no peace in Syria as long as Iranian forces remained there.

“Iran is arming itself and its forces against Israel, including from Syria territory, and is, in fact, gaining a foothold to continue the fight against Israel,” he said following his meeting with Mr. Putin.

Plans for a naval base

Iran seems to have no intention of abandoning Syria. In November, Gen. Mohammad Hossein Baqari, the chief of staff of the Iranian armed forces, said the Islamic Republic could in the future establish naval bases in Yemen and Syria. Iran is providing some military support to Houthi militiamen, who with their Yemeni allies have been battling a Saudi-led military coalition since March 2015.

Mr. Netanyahu raised the Iranian naval base proposal during his meeting with Putin. Moscow’s immediate response to Netanyahu’s concerns is so far unclear, but many analysts doubt that Russia will – or can – apply pressure on its Iranian battlefield ally to reverse its agenda in Syria.

Although Assad and his Russian, Iranian, and Lebanese allies have gained the upper hand in the conflict, the war is far from over. Given the chronic manpower shortage facing the Syrian Army, Assad and his partners continue to need each other to prevail, a reality that weighs against an unraveling of the alliance for now.

"The possibility of working with Russia in an attempt to minimize Iranian influence is being examined,” says Frederic C. Hof, the director of the Rafik Hariri Center for the Middle East at the Atlantic Council in Washington. "As of now there is no evidence of Moscow's willingness to do so and no evidence that it has the leverage to restrain Assad or Iranian-led militias.

“Russians are telling members of the Syrian opposition that they are fed up with the unprofessionalism of Assad's army and the Shiite militiamen brought in by Iran from Lebanon, Iraq, and Afghanistan," Mr. Hof continues. "But it's not clear what Moscow would be willing and able to do about it."

Israel’s diplomatic attempts to block an Iranian naval presence in Syria may have come too late. On Monday, Russia’s Nezavisimaya Gazeta daily reported that Assad has already green lighted the establishment of an Iranian naval base, which is to be located on the coast close to Jableh and the Hmeimim air base, which is currently being used by the Russian Air Force.

If the report is confirmed, a naval base in Syria potentially grants Iran a maritime route to transfer armaments to Hezbollah in Lebanon in addition to the long-established air route via Damascus airport. It could also create friction in the waters of the Mediterranean, which is patrolled by the Israel Navy and the US Navy’s Sixth Fleet.

Building up the Golan front

The other area of interest for Iran in Syria is the Golan Heights, the volcanic plateau in southwestern Syria that overlooks northern Israel. Much of it was seized by Israeli troops in the 1967 war and has since remained under Israeli occupation. Hezbollah fighters and Islamic Revolutionary Guard Corps (IRGC) personnel have had a presence on the northern Golan since at least late 2013.

According to sources close to Hezbollah, the Lebanese group in 2014 installed some military infrastructure in the area north of Quneitra, including bunkers and firing positions, with the goal of extending the front with Israel from south Lebanon into the Golan. In January 2015, an IRGC general and six Hezbollah members, including two senior officers, were killed in an Israeli drone strike near Quneitra. The team had been inspecting the newly-installed facilities when they were attacked, say sources close to Hezbollah.

Between 100 and 150 Hezbollah fighters are deployed in the northern end of the plateau, and an unknown number of IRGC personnel are suspected to be with them, say Western security sources familiar with the dynamics on the Golan. Currently, they and other government forces are in a standoff with anti-Assad rebels in the area. However, the Assad regime is negotiating a series of cease-fire agreements with communities in the northern Golan in which rebels are allowed to move to the Idlib province in the north in exchange for the resumption of state control in the vacated villages.

If the process continues to be successful, the northern Golan may become neutralized from the Syria conflict, which could allow Iran and Hezbollah to revive its original plans for the area. In a possible signal of Iran’s future intentions, the Hezbollah al-Nujaba Movement, an Iran-backed Iraqi Shiite militia, announced last week that it was establishing a new unit called the Golan Liberation Front.

“This is a trained army with specific plans,” said Sayyed Hashem Moussawi, the leader of the group, in a press conference announcing the new group. “If the government of Syria requests, we and our allies are ready to take action to liberate the Golan.”

Since the last war between Hezbollah and Israel in 2006, the border between Lebanon and Israel has remained mostly calm, with both sides wary of triggering another, more devastating, conflict. But, says Giora Eiland, a former Israeli national security adviser and retired general, unlike Lebanon, where Israel can hold the Lebanese government accountable for Hezbollah actions from its territory, the Syrian government has limited control over its land.

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"Syria is far from a state that can be accountable, and if Hezbollah manages to create positions on the Israeli-Syrian border and act against Israel from Syria, it would be very hard to deter the Syrian government,” he says.

Correspondent Joel Greenberg contributed from Jerusalem.