The US, UK and France are set on a collision course with Russia over possible strikes on Syria after another suspected Syrian poison gas attack - and it could lead to "direct war".
US President Donald Trump has declared that missiles "will be coming" towards the Syrian military, and Waikato University international relations expert Alexander Gillespie warns a strike is "quite likely".
- Trump warns Russia that missiles are 'coming, nice new and smart
- UK sends submarines to Syria, ready to strike within 24 hours
"Trump has promised that he will do this, and he appears to be trying to let the Russians know it is coming so they are not overtly startled when it does," he told Newshub.
"Apart from having promised he would do so, he has acted similar in the past with a missile strike around the same time last year into Syria.
"If he fails to act he will be guilty of the same weakness he accused Obama of - for not acting after threatening repercussions for crossing a red line; and finally, he needs to look super-strong as he goes into his negotiations with North Korea."
The Russian military says it has observed movements of US naval forces in the Gulf. British Prime Minister Theresa May has also reportedly ordered British submarines to move within missile range of Syria in readiness for strikes, according to the Daily Telegraph.
Russia warns that any missiles fired at Syria would be shot down and the launch sites targeted.
"If there is a strike by the Americans, then... the missiles will be downed and even the sources from which the missiles were fired," Russian Ambassador Alexander Zasypkin told Hezbollah's al-Manar TV.
The dramatic rise in tension has led to fears of military conflict between the two giants. Prof Gillespie warns Russia is in a "very belligerent mood" and has had enough of being pushed around.
"They have spoken very clearly that they will intervene," he told Newshub.
"Russia does not tend to issue idle threats. If they shot down the missiles, that is one thing. If they go after the launch sites - as in the planes, boats, subs or land based - we are at risk of a direct war."
Top US officials agree, saying that Russian leader Vladimir Putin is likely to respond.
"Putin will see this as a test of wills, given Trump's tweet, no doubt about it," former US National Security Council member Michael Carpenter told Politico.
"I would expect Russia to try hard to shoot down cruise missiles and claim this as a victory," said another expert.
Professor Gillespie said this represents an "insane risk".
"Right now, Trump has promised to strike, and Putin has promised to respond. One of them has to back down, or things are going to get very dangerous."
It would be the second strike Mr Trump has ordered in retaliation for a chemical attack by Syrian leader Bashar al-Assad's regime.
But any strike that killed official Russian forces would be a dangerous escalation.
US forces killed over 200 mercenaries, mostly Russians, in Syria in February during a failed attack on an American base in Syria. They were reportedly sent to the conflict by a Putin ally in exchange for oil concessions.
However the Russian government denied any involvement in the attack, blaming it on a "pro-government militia unit".
But with uniformed Russian soldiers directly assisting President Assad's forces at most key Syrian military targets, a strike by Western forces that killed one would need to be acknowledged by Mr Putin - possibly leading to retaliation.
"If Russia decides to respond with force it might also be against American forces on the ground in Syria - where there are at least 2000 soldiers," Prof Gillespie said.
"These could be targeted too. Or, in true Russian style, they could pull the strings in some other part of the world and cause conflict there."