US ships loaded with rockets at €670,000 apiece
Donald Trump's promise to carry out US strikes against Syria despite Russian warnings could result in a showdown between two of the world's most sophisticated weapons systems.
US action against Bashar al-Assad will almost certainly come in the form of a hail of Tomahawk missiles, the sea-launched weapons costing €670,000 each.
But that strike could be disrupted, if not entirely thwarted, by Russia's state-of-the-art but untested S-400 air defence system.
Launched from US Navy ships and Royal Navy submarines, the Tomahawks can deliver a 1,000lb (450kg) warhead with pin-point accuracy from ranges of 1,200 to 2,400km, flying at 550mph just metres above the ground. But they have never been challenged by an air defence system as modern or sophisticated as the S-400, which Russia deployed to its Hmeymim airbase in Syria in 2015.
The S-400 has a sophisticated radar and control array that allows it to target dozens of enemy aircraft simultaneously at ranges of up to 400km.
While its missile interceptor capability is shorter range - about 120km - its missiles travel at 1,000 metres per second and can hit low-flying targets at just a few metres of altitude - perfect for killing sub-sonic Tomahawks.
US commanders, however, may plan to overcome that impressive hit rate with an overwhelming number of Tomahawks, given that Western experts estimate the Hmeymim system has only around 60 missiles.
The S-400 is not the only defence coalition commanders have to worry about.
Russia is also believed to have missile cruisers carrying the older S-300 anti-aircraft system off the coast and defending its base at Tartus. It also has a number of SU-30 interceptor aircraft in Syria, which could pose a serious challenge to coalition pilots.
Meanwhile, the Syrian Arab Army fields a formidable array of older, mostly Soviet-designed surface-to-air missiles, which while little threat to cruise missiles could prove extremely dangerous to coalition aircraft.
That effectively rules out conventional airstrikes, which would risk the lives of the coalition pilots and gravely raise the risk of direct conflict between Russian and Western forces.
Other coalition options include Britain's Storm Shadow cruise missile, an air-launched long range "stealth" cruise missile designed to evade radar that could be fired by RAF Tornadoes flying out of Cyprus.