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Since the spring of 2017, there is evidence of Iranian construction of a land route for the traffic of military forces and weapons from Iran, through Iraq, to Syria and Lebanon. This route, called "the corridor," was supposed to be under Iranian control and influence by means of the Revolutionary Guards and the armed Shiite militias. Iran has so far not announced its intention of building this land bridge, but it would be a logical means for rapid movement to Syria and Lebanon of both large fighting units of the Revolutionary Guards and the Shiite militias and of high quality weaponry. This route seems more secure, complementing or replacing air transport from Iran to Damascus, and thus diversifying the methods of moving troops and arms, according to conditions on the ground. The use of the corridor is probably intended above all for the movement of troops to Syria to further assist in stabilizing the Assad regime. However, at a later stage, or in parallel, the corridor can strengthen Hezbollah’s capabilities against Israel – and perhaps those of other Shiite militias currently stationed in Syria – and thereby intensify Iranian deterrence and the threat to Israel.

How the corridor will be used by the Iranians and their proxies is not yet clear. It appears, however, that to date such use is limited. So claimed the US Secretary of Defense James Mattis, who in early January 2018 said that so far Iran had not yet managed to build a land bridge through Syria to the Mediterranean, and that at this stage such a bridge did not exist. According to Mattis, construction of the route was delayed by the ongoing fighting against the remnants of the Islamic State and other forces still in the area. Iraqi sources – whose reliability is unknown – claimed that in mid-December 2017 the first Iranian military convoy passed from Iraq to Syria, toward Deir ez-Zor. The convoy consisted of 20 trucks, carrying unidentified cargo, and was escorted by Revolutionary Guards and Iraqi militias. In the assessment of these sources, this apparently marked the launch of the operation of the land bridge from Iran to Syria.

The statements from Mattis and the Iraqi sources indicate that so far, the corridor has not been used to a significant extent, and the movement of troops and weapons from Iran to Syria remains principally by air. The delay in operating the corridor is likely due not only to the continued fighting in some of the areas, but also to the Iranians' fear of attacks on convoys by the United States and/or Israel, as well as by Sunni elements, the Islamic State, and organizations opposed to Assad. These fears may be heightened if the Iranians realize that moving troops and materiel by land gives Israel more time to take action against the convoys.

Moreover, on January 20, 2018, US Secretary of State Rex Tillerson announced that one of the goals of the United States in the Middle East is to destroy the Iranian dream of moving freely from Iran to the Mediterranean. For this purpose, the US will maintain its military presence in Syria for a period of unknown duration, focusing on preventing the return of the Islamic State to the area, and acting to block Iranian influence in Syria. Tillerson said that the administration would try to use another tool for limiting Iranian influence in Syria – striving to set up a Syrian government without the involvement of Assad and his regime. Underlying this effort is the expectation that if such a government is established, it would seek to rid itself of the Iranian presence and its dependence on Tehran.

There is no doubt that the United States’ announcement of its intention to maintain a military presence that has no defined end date should worry the Iranians. It will hamper their long term attempts to build a considerable Shiite military presence in Syria, including Hezbollah, with the assistance of limited Iranian forces. In particular the Iranians should be worried by the administration's declaration that its military presence – despite being smaller than the Iranian presence – is explicitly intended to limit Iran's influence in Syria. They should also be worried by the US intention to strengthen, with the assistance of its allies in Syria, the Syrian Democratic Forces and the Kurdish militias. These are intended to take control of northeastern Syria, if it can survive the Turkish pressure, and thus also threaten the Iranian corridor from Iraq to Syria.

Thus far President Trump has threatened to limit Iran's stature in the Middle East, but many of his promises have not been fulfilled, as he has been unable to assemble the tools required to block Iran. There is also no certainty that the administration is actually prepared to confront Iranian / Shiite troops and Hezbollah, in view of the painful memory of United States intervention in Iraq in the previous decade. Indeed, the US intervention in Iraq has been criticized by Trump himself, who declared that he would not have sent American troops to Iraq.

For its part, Iran has no interest in military involvement with US forces, and it does not know what Trump will do to achieve his objectives. Iran must also consider that Russia might be prepared to reach a settlement that does not meet all of Iran’s interests, is based on agreement with the American administration, and above all does not ensure the continuation of the Assad regime. Such a settlement could seriously hurt Iran's interests and influence in Syria and the region, including the operation of the corridor.

Because of the uncertainty surrounding the American response to the corridor, along with the fear that Israel could attack convoys passing through it, Iran will probably be cautious about exploiting this land route. Accordingly, Iran will likely examine the red lines of the US and Israel, and the extent of their response to convoys along the corridor. There is a strong chance that Iran will restrain itself and not respond to American attacks. It is also more likely to use the route for Shiite militias rather than large numbers of Iranian troops, as this will give it more flexibility, avoid heavy losses of Iranian fighters, and mean that it is not obliged to respond to attacks. Presumably Iran will not use the corridor continuously, but will prefer to send convoys from time to time, subject to the responses of the United States and Israel.

The possibility that Iran will use the corridor increases the threat to Israel, because it could be a pipeline for reinforcing Hezbollah and because it helps the military deterioration of the situation between Iran and Israel. However, the fact that until now Iran has not rushed to operate it, the assumption that it may be cautious about using it and that any use will be limited, and that the American administration has announced that its military presence in northeast Syria will continue for an undefined period could reduce the seriousness of the threat.

Iran has never publicly announced its intention to operate the corridor, and thus has maintained its room to maneuver on this issue. Presumably Iran recognizes that using this land route has advantages as well as disadvantages relating to its exposure to attacks by the United States, Israel, and Sunni elements, and to the US military presence in northern Syria. To the extent that Iran assesses that these disadvantages outweigh the benefits, it is likely to limit its use of the corridor  and vice versa. The Iranian decision will be largely influenced by the reactions of the United States and Israel.