Iran could soon attain a land bridge to the Mediterranean Sea for the first time in 1,400 years. The Sassanid Empire, with its capital in southern Iran, had direct access to the sea in the early 600s. No Iran-based government has had access since then.But an Iranian route to the sea might reappear in the coming months, presenting Israel with its greatest strategic threat since the Yom Kippur War.Iranian proxy militias have multiplied in Syria since Hezbollah first intervened to help the Assad regime defeat a popular uprising in 2013.These militias include the Iraqi Nujaba Movement, whose leadership pledged allegiance to Iran’s supreme leader; the Afgani Fatimiyoun and Pakistani Zaynebiyoun, both under direct Iranian Revolutionary Guards command; and numerous militias from the Iraqi Popular Mobilization Forces.As Assad’s position has stabilized, these militias are turning their sights on Israel. The Nujaba Movement formed a Golan brigade in March, vowing to attack Israel “should the Syrian government make the request.”Hezbollah leader Hassan Nasrallah threatened in June that “thousands...from Iraq, Yemen, Iran, Afghanistan and Pakistan” would join the next war with Israel. Since Assad today is a junior partner to Iran’s foreign proxies in Syria, victories for the pro-regime side in fact place territory under Iranian control.Assad and Iran are only months from completing their corridor. Along three axes, they are closing in on the eastern city of Deir ez-Zor, whose capture would make the corridor almost inevitable. Yet neither Israel nor the US has prioritized this threat, focusing instead on Israel’s border. The IDF last week revealed the scope of its impressive aid efforts to villages on the Syrian Golan. American President Donald Trump recently arranged a cease-fire with Russian President Vladimir Putin in southwestern Syria, which includes the Israeli and Jordanian borders. But eastern Syria is today even more strategic for Israel than the Syrian Golan. An Iranian corridor through Syria would help Iran to sneak “game-changing” weapons into Lebanon while evading Israeli detection. It would enable Iran-backed extremists to more easily reach the Israel-Lebanon border. Over time, Iran would acquire a Mediterranean port, allowing for the harassment of Israeli ships.That port would also increase Iranian trade with Europe, complicating efforts to sanction Iran even for nuclear violations.Trump’s recent cease-fire deal with Putin exemplifies the flaws in current Israeli and American thinking. With respect to the Syrian Golan, the deal was largely successful. It cemented a “de-escalation zone” and froze battle lines with the Golan under rebel control. It required that Iranian proxies leave southwestern Syrian, to be replaced by Russian or other forces.But on a broader level, the deal failed.First, the deal eased pressure on Iran by freezing fighting in an area where pro-regime forces were losing. In the weeks before the cease-fire, pro-regime forces had launched multiple failed offensives in southwestern Syria; they managed only to take heavy casualties and draw more rebels into the fight.Second, the cease-fire enabled Iran by excluding rebels trained in America’s Tanf base near Jordan. In late April, these rebels were gaining rapidly against Islamic State and seemed on the cusp of blocking Iran’s corridor.Russia rescued Iran in May, with an initial “de-escalation zones” plan that included all opposition areas except Tanf. This enabled the regime to redeploy elite forces toward Tanf on the day of the cease-fire, reinforce Iranian proxies, and ultimately attack and outflank the Tanf rebels. Yet despite this history, the Trump administration compounded the problem by arranging a cease-fire that excluded Tanf once again. Since the cease-fire, Tanf rebels have suffered further massive losses.The chances of blocking an Iranian corridor are now slim. An American anti-ISIS spokesman has “welcomed” a potential Iranian conquest of eastern Syria because “we are not in the land-grab business.” President Trump has ended covert CIA support to Syrian rebels, which stands to profoundly weaken rebels in southwestern Syria. The Syrian Democratic Forces (SDF) still benefit from overt Pentagon support, but they are too busy fighting in Raqqa to open a new front.Israel and the US must recognize the dangers of an Iranian corridor and work urgently to block it. While it is theoretically possible to target Iranian bases after a corridor is complete, American appetite for this is limited, and such a move would leave Israel diplomatically exposed. And even airstrikes would not work if Iran eliminated its anti-Assad adversaries; Iranian proxies would simply rebuild, just as Hezbollah has in south Lebanon.The only surefire way to block an Iranian corridor is to help anti-Iran forces seize ground along the corridor before Iran defeats them. In particular, the Pentagon is now in talks with Tanf rebels for the rebels to redeploy to northeastern Syria and fight ISIS from SDF front lines. Such an option is likely the last chance to block an Iranian corridor without fighting Iran directly.It would combine the Tanf rebels’ anti-Iranian record (a record the SDF does not possess) with the heavy air support that only the SDF has received so far. America and Israel should fully support this option.More broadly, the US and Israel should stand up to Iran’s campaign of war crimes in Syria by empowering the moderate rebels who want Iran out of their country. Israel must turn its gaze beyond the Golan; the US must rethink its growing apathy to the fight against Assad. Syrians deserve international support against their killers – otherwise, we might soon awake to a terrifying geopolitical reality in which Iran is stronger than ever.The author is the policy and advocacy officer at the Syrian American Council.