BEIRUT, Lebanon — When an Iranian drone flew into Israeli airspace this month, it set off a rapid series of strikes and counterstrikes that deepened fears over whether a new, catastrophic war was brewing in the Middle East.

That flare-up ended quickly, if violently, with the drone destroyed and an Israeli jet downed after bombing sites in Syria. But the day of fighting drew new attention to how deeply Iran has embedded itself in Syria, redrawing the strategic map of the region.

Tactical advisers from Iran’s Islamic Revolutionary Guards Corps are deployed at military bases across Syria. Its commanders regularly show up at the front lines to lead battles. Iran has built and continues to back powerful militias with thousands of fighters it has trained in Syria. And it has brought in new technologies, like drones, to spy on enemies and perhaps to attack them from the sky.

Both Israeli officials and Israel’s enemies say that any new conflict between Israel and Iran, or any of its allies, could mobilize Iran’s expanding network of militant proxies in multiple countries, what Iran refers to as “the axis of resistance.”

“If there is a war, it will be regional,” said Kamel Wazne, the founder of the Center for American Strategic Studies, in Beirut, who studies the policies of the United States and Iran in the Middle East. “Any confrontation will be with the whole resistance front against Israel and its backers.”

Iran and its allies first intervened in Syria to defend the rule of President Bashar al-Assad against Syrian rebels after the civil war broke out in 2011, and later helped his forces against the jihadists of the Islamic State.

But as the rebels have lost ground and no clear threats to Mr. Assad’s rule remain, Iran and its allies have stayed, shifting their focus to creating an infrastructure to threaten Israel, analysts say. Iran continues to train and equip fighters while strengthening ties with allies in Iraq and Lebanon, in hopes of building a united front in the event of a new war.

“The ultimate goal is, in the case of another war, to make Syria a new front between Israel, Hezbollah and Iran,” said Amir Toumaj, a research analyst at the Foundation for Defense of Democracies who studies Iran. “They are making that not just a goal, but a reality.”

Iranian leaders speak openly of their work to build this axis of resistance against Israeli and American influence. A key to Iran’s strategy, analysts and officials say, is to rely not on conventional military hardware or control of territory, which Israel can easily bomb, but on building ties with local forces who share its goals and benefit from its financing and expertise.

That approach has enabled Iran to amplify its power in the Arab world while decreasing the threat to its own forces and homeland. It has also created a problem for countries including the United States, Israel and Saudi Arabia, who fear Iran’s growing influence but have struggled to come up with ways to stop it.

Some people in Israel have started referring to a potential “First Northern War,” meaning that Israel will have to fight across both the Lebanese and Syrian frontiers. And many Israelis say the danger is not just from the new Iranian-backed militias, but also from the Iranian efforts to give advanced, high-precision weapons capable of hitting sensitive infrastructure to Hezbollah, Iran’s most powerful and experienced external force.

Israeli officials have said that Iran and its allies are seeking to establish a land corridor from Iran to the Mediterranean, via Iraq, Syria and Lebanon, to ease the transportation of such weapons and to build underground factories to manufacture them in Lebanon and Syria. Israel has been bombing convoys in Syria that are believed to be carrying advanced arms to Hezbollah, but the group’s covert nature makes it hard to determine which arms have slipped through and whether its arms factories are functioning.

Such arms, coupled with heavy barrages from the more than 100,000 rockets and missiles without high-precision targeting capability that Israel says Hezbollah already has, could overwhelm Israel’s defenses.

“Israel will face not only quantity, but the threat to vulnerable strategic sites,” said Yaakov Amidror, a former Israeli national security adviser and now a fellow at the Jerusalem Institute for Strategic Studies. Referring to the combination of more precise weapons and a new front, he added: “Each one is problematic; together, they are devastating.”

Iran’s moves in the region have alarmed the United States. “What’s particularly concerning is that this network of proxies is becoming more and more capable as Iran seeds more and more” of its “destructive weapons into these networks,” Lt. Gen. H.R. McMaster, President Trump’s national security adviser, said at a security conference in Munich on Saturday. “So the time is now, we think, to act against Iran,” General McMaster added.

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