On the evening of April 6, the United States sent 59 Tomahawk missiles slamming into the Shayrat air base in Syria in retaliation for the unconscionable gassing of dozens of civilians by the Bashar al-Assad regime two days earlier. This was a stunning turnaround for President Donald Trump, who had urged his predecessor not to attack Syria under similar circumstances in 2013, and for an administration that had signalled indifference to Mr Assad just five days before the hit.

Mr Trump's forceful action met with rave reviews across Washington and among US allies. But the Russians - Mr Assad's chief patron - were furious. The Kremlin called the attack an "act of aggression", and Prime Minister Dmitry Medvedev warned that the strikes put the US and Russia "on the verge of a military clash". Meanwhile, the Russian military suspended the hotline designed to avoid incidents between US and Russian forces in Syria and announced its intentions to bolster Syrian air defences.

However justified and morally satisfying, any use of military force is serious business, and even last Thursday's limited strikes could lead the US and Russia down an escalatory path.

The expansive way in which US officials have talked about the purpose of the strikes increases the prospects of mission creep. In his statement announcing the attack, Mr Trump framed it as essential to "prevent and deter the spread and use of chemical weapons". Other administration officials justified the strikes on similar grounds. But Mr Trump also described his decision as part of a broader effort to "end the slaughter and bloodshed in Syria", suggesting that he may consider more military action aimed at ending the Syrian war. The US ambassador to the United Nations, Ms Nikki Haley, made this explicit, saying last Friday: "We are prepared to do more, but we hope that will not be necessary. It is time for all civilised nations to stop the horrors that are taking place in Syria and demand a political solution."

The broader the US administration's goals in Syria, the more prone it will be to pressure to escalate there. Already, some regional allies that have long dreamed of dragging the US into a war with Mr Assad, such as Turkey, have described the April 6 strikes as "insufficient" and called for more forceful action. And congressional hawks like senators John McCain and Marco Rubio are urging Mr Trump to follow up on the strikes by providing Syrian opposition groups with more weapons, imposing a no-fly zone and conducting further air strikes in order to pressure Russia and Mr Assad to agree to a political settlement.

Meanwhile, the Syrian air force has already resumed bombing the north-western town of Khan Sheikhoun - the very same area the regime had earlier gassed. And as Mr Assad continues to kill civilians, with or without chemical munitions, the calls for deeper US involvement aimed at ousting the Syrian leader will mount.

If Washington goes down this road, the prospects of a military confrontation with Moscow are real. A few thousand Russian military personnel are distributed across Syria's key military bases. Moscow has also placed some of the world's most sophisticated air defence systems in Syria, and Russian planes police Syrian skies. So an extensive US campaign aimed at coercing Mr Assad by targeting Syrian air bases and command-and-control facilities would run big risks of killing Russian troops on the ground. The same holds for a no-fly zone, which would likely require targeting Syrian and Russian air defences and could lead to air-to-air incidents between Russian and US jets. Under any of these circumstances, the prospect of spiralling conflict is enhanced by Moscow's decision to suspend the "deconfliction" channel between the Russian and US militaries.

Compounding matters, in an effort to generate counter-leverage, Russia and the Assad regime are likely to respond to further US strikes or a no-fly zone by reorienting their integrated air defence network towards US and coalition aircraft engaged in fighting the Islamic State in Iraq and Syria (ISIS), or by attacking opposition areas in northern Syria where nearly 1,000 US troops are on the ground. This could derail the counter-ISIS campaign at the very moment when the ISIS capital, Raqqa, is under assault by US-backed forces. And it could put the lives of US service members in jeopardy.

There are also significant escalation risks even if the Trump administration doesn't go down this path and sticks to the narrower stated objective of deterring further chemical attacks.

According to news reports, before Thursday's strikes, Mr Trump was briefed on a number of options to retaliate against Mr Assad. One option was to launch "saturation strikes" aimed at dozens of Syrian airfields and facilities, with the goal of destroying Mr Assad's ability to use his air force to carry out further chemical attacks. (This option was similar to the one reportedly contemplated by then President Barack Obama in 2013.) At the urging of Defence Secretary Jim Mattis and national security adviser H.R. McMaster, Mr Trump rejected this option in favour of cruise missile strikes against a single air base. This option was reportedly viewed as proportional and much less likely to kill Russian soldiers. (Although around 100 Russian troops were reportedly stationed at Shayrat, the administration warned Moscow ahead of time, and the missiles struck the part of the airfield away from Russian barracks.) In other words, Mr Trump chose a limited strike package precisely because he and his advisers understood the grave risks if the US attacked a broader set of targets.

But therein lies a major dilemma for Mr Trump moving forward. Successful deterrence requires a credible threat to hit Mr Assad's forces again and again if the regime continues to use chemical weapons or commits other transgressions. Yet Mr Trump, having already rejected a larger military response out of apparent recognition of the dangers, may find it hard to credibly signal he is willing to use this very option in response to further actions by Mr Assad later.

In this context, the danger of miscalculation is real. The Syrian dictator (perhaps prodded by Russia or Iran) may attempt to test Mr Trump again, hoping to prove him a "paper tiger". And Mr Trump, having invested his personal credibility in standing firm, may find himself psychologically or politically compelled to respond, despite the very real risks that it could result in a direct military clash with Russia.

Before Mr Tillerson arrives in Moscow for meetings tomorrow, the administration needs a clear plan to avoid stepping on this slippery slope. It starts with Mr Trump and his team being much more precise and consistent about what US objectives are. Is the goal solely to deter more chemical attacks, or are they trying to end the Syrian conflict? If the latter, will the administration insist on Mr Assad's departure, or are they open to other possible formulas that de-escalate the war and defuse power away from Damascus but keep Mr Assad in power?

On Sunday, administration officials seemed to suggest all of the above. Ms Haley insisted on CNN: "There's not any sort of option where a political solution is going to happen with Assad at the head of the regime." But Mr Tillerson warned against regime change during an ABC "This Week" interview, and told CBS' "Face the Nation" that the US administration would seek to end Syria's civil war through the creation of ceasefire zones and the resumption of a political process, saying Mr Assad and Russia would have to participate in that solution.

Contributing to this uncertainty, Mr Tillerson also said the US would not focus on initiatives to stabilise Syria until after the ISIS threat has been "reduced or eliminated". Whatever the administration decides, its approach must blend credible military signalling with risk-mitigation steps (like finding a way to reactivate the US-Russia military channel). And the US administration's military actions must be backed by a diplomatic strategy that takes advantage of the leverage created by last Thursday's strikes without overplaying Washington's hand.

Given Russia's vital interests in Syria, Moscow is not likely to respond positively to US ultimatums and maximalist positions. If the US administration does not find a way to give the Kremlin a face-saving way out, conflict is much more likely than accommodation.