Editor's Note: (Ranj Alaaldin is a Visiting Fellow at the Brookings Doha Center and specializes in Shia and Kurdish movements in the Middle East. He is the author of a forthcoming book on Shia militias in Iraq and Syria. The opinions in this article belong to the author. )
(CNN) The Syrian regime's use of chemical weapons has been met with a decisive response from the US.
Russia denounced Thursday's airstrike on the Shayrat airfield, but is unlikely to counter-escalate, while the international community has generally responded positively.
Whether the Trump administration will escalate its involvement in Syria remains unclear, but if it decides to do so, then it should also prepare for a confrontation with Iran.
It is Iran and its proxies, not Russia, that dominate realities on the ground, and it is from the ground that Syria's future will be determined and where the post-conflict environment will be shaped.
Nikki Haley, the US Ambassador to the UN, has spoken of discarding the Iranian presence and influence in Syria.
However, Iranian influence cannot be eliminated in Syria, as there is not a single Iranian proxy to target, but rather an extensive network of leaders, fighters, resources and weapons that span across the region and into Afghanistan.
Since the Iranian regime entered the Syrian conflict in 2012, a plethora of Iranian-backed militia groups have consolidated their hold on the ground. Through these groups, the balance of power has shifted in the regime's favour and, in the process, to their Iranian patrons.
This should come as no surprise. Iran has spent almost four decades nurturing and consistently dedicating resources to armed groups that could either enhance the regime's influence in the region or, at the very least, weaken its rivals.
Iran's clerical rulers, since their assumption of power in 1979, have established and worked with armed groups across the Arab world.
In the 1980s, the Iranian regime established Lebanon's Hezbollah, along with Iraqi Shia insurgent groups who fought the Baath regime -- some of whom today constitute Iraq's most powerful militias and political actors.
It has provided support to components of the Palestinian movement, such as Hamas and Islamic Jihad.
In the 1990s and early 2000s, jihadi groups such as Ansar al-Islam -- the previous incarnation of Al-Qaeda in Iraq and so called Islamic State -- also used Iranian territory as a launching pad for terrorist attacks in neighbouring Iraqi Kurdistan.
Iran's investments are paying dividends.
While many were not prepared for the emerging regional order, in which armed groups are the providers of security or dominate the local environment because of conflict and state fragility, Iran has decades' worth of experience in trying, testing and learning from the proxy warfare that is currently engulfing the Arab world.
Lebanon's Hezbollah has played a decisive role in Syria since it captured the strategic town of Qusayr in 2013. Iraqi Shia militias have flooded to Syria to fight alongside the regime, many of them battle-hardened from their experience fighting American and British personnel in Iraq.
The Shia militias that Iran established in Iraq were organized and financed by Tehran; they were given their own identities and, therefore, the chance to acquire their own local resources and support bases.
These groups have become integrated into Iraq's political system and now have affiliates in Syria that are firmly entrenched in the areas they control.
These affiliates have also started to establish local support and support from Shia throughout the region who fear that the overthrowing of Assad constitutes an existential threat to the Shia as a whole, fears that Iran exploits.
Iran's influence in Syria, therefore, has a cultural and religious dimension, which adds to the complexity of combating it. Rather than focusing on defeating Iran in Syria, the US must focus on containing it.
It started to do that on Thursday: America's presence reassures its friends and worries its enemies. While their confidence may have been high before, Iran and its proxies will now be looking over their shoulders in Syria.
Countering Iran effectively will require long-term approaches to a complex and constantly evolving problem. The US must match the dedication and resources that Iran has invested: that means establishing a foothold in Syria.
Establishing the safe zone that the Trump administration is considering and that its allies have called for will effectively constitute a buffer zone against Iranian influence. It is no longer simply a moral imperative to pursue such a zone but also a strategic imperative.
Syria's future will most likely be a federal model where power is devolved to different regions. Iran's strategy for Syria is accordingly set out in a way that allows it to dominate, such governing structures.
It should come as no surprise if Iran attempts to integrate its proxies and the territories they currently control into the post-conflict political system and institutionalize them into the state, as it has done with its proxies in Iraq.
Whether it is on the ground or at the negotiating table, the US can leverage its presence in Syria to prevent Iran from attempting to institutionalize its proxies.
A safe zone that acts as a buffer against Iranian influence will enable that and ensure Iran does not have a free-hand in shaping Syria's future.
While Moscow has influence in Syria, its hands are tied. From the Iranian perspective, it would be counter-intuitive to withdraw militias who provide it with unparalleled influence and these militias, in any case, far outnumber and outweigh the forces that Russia has deployed on the ground.
However, the Assad regime and Iran are becoming increasingly assertive and have started to move away from Russia's sphere of influence.
That presents an opportunity for the US to coordinate its actions with the Russians as it attempts to contain Iran's influence in Syria.