On Monday afternoon, as a policeman’s coffin follows the route of a home-grown murderer’s speeding car, it appears Theresa May faces that same eerily familiar choice that once faced Tony Blair. Except this choice is a hundred times more complex.

An internationally unpopular US President appears to be on the brink of escalating action in the Middle East. Both preemptively and retrospectively, the UK has given every indication of its intention to stand by its ally. In her speech in Philadelphia the day before meeting Donald Trump at the White House, Theresa May told the new President they would “pick up that mantle of leadership once more”. And in the hours after 59 US Tomahawk missiles landed on a Syrian government air base, the UK Defence Secretary, Michael Fallon, said President Trump had “made the right call by resorting to careful and narrowly focused military action”.

And now, the Foreign Secretary Boris Johnson has cancelled his planned visit to Russia, allowing the US Secretary of State Rex Tillerson, a former Russia-based oil executive and genuine friend of Vladimir Putin, to go alone and deliver a message on behalf of the West.

But what happens next? The US ambassador to the UN has been clear that the US is “prepared to do more” if the international red line on the use of chemical weapons is crossed again. Meanwhile, a joint statement from Russia and Iran talk of their own “red lines” and their willingness to respond “with force".

In pictures: US missile strike against Syria Show all 7 1 /7 In pictures: US missile strike against Syria In pictures: US missile strike against Syria The guided-missile destroyer USS Porter (DDG 78) launches a tomahawk land attack missile in the Mediterranean Sea AP In pictures: US missile strike against Syria The United States military launched at least 50 tomahawk cruise missiles at al-Shayrat military airfield near Homs, Syria, in response to the Syrian military's alleged use of chemical weapons in an airstrike in a rebel held area in Idlib province EPA In pictures: US missile strike against Syria Shayrat airfield in Syria Getty Images In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria US Navy guided-missile destroyer USS Ross (DDG 71) fires a tomahawk land attack missile in Mediterranean Sea Reuters In pictures: US missile strike against Syria President Donald Trump speaks at Mar-a-Lago in Palm Beach, Fla., after the US fired a barrage of cruise missiles into Syria in retaliation for this week's gruesome chemical weapons attack against civilians AP In pictures: US missile strike against Syria Syria's President Bashar al-Assad Reuters

“What America waged in an aggression on Syria is a crossing of red lines,” the two countries said. “From now on we will respond with force to any aggressor or any breach of red lines from whoever it is and America knows our ability to respond well.”

It is, frankly, terrifying stuff.

When Tony Blair decided in 2001, that action must be taken in Afghanistan against the government that had given succour to the terrorists who had perpetrated a truly devastating multiple attack on US territory (which also carried the highest British death toll of any terrorist attack in our country’s history), he calculated that, on the morning of 11 September 2001, the world had changed; that in order to exercise his primary duty – namely, ensuring the safety of the citizens of his country – action was required.

The 2003 decision, to stand shoulder to shoulder with Bush in Iraq, is far more complex and far more contested. But even on his own admission, he placed immense weight on the preservation of the alliance with the United States. That he would “be with you whatever”.

Of course, 9/11 was the most traumatic incident the West had faced in decades. Blair acted almost without precedent. But the question now facing Theresa May is more complex than anything Blair ever had to consider, and meanwhile she drowns in precedent.

Her would-be master is unpredictable, impulsive even. He won an election alternating between promises first to destroy Isis and secondly not to get involved in Syria. Then, it would appear, images of gassed Syrian children have persuaded him to launch action against Bashar al-Assad, an act of (albeit proportioned) aggression against a regime allied with Russia and Iran.

Syria: What led to the US airstrikes?

To appear to be at Trump’s side whatever the cost is the sort of bet-the-house gamble that leaves punters peeping at the finish post through the gaps in their fingers.

Yet other spectres hover over it. At Trump’s right side, quite literally, in those Mar-a-Lago "situation room" photographs from last week – and almost unremarked upon – was Wilbur Ross. He is the distressed-debt billionaire who will call the shots on any free trade deal a freshly Brexited UK might get with the US. Distressed-debt specialists all do the same thing. They are Richard Gere in Pretty Woman. They take everything there is to take from near-bankrupt companies who’ve got no choice but to accept what they’re given.

Years after the invasion of Afghanistan, certain types of commentators liked to make the point that al-Qaeda had been "given what they wanted". That they had agitated for a war that would itself be the recruiting sergeant to their cause. I find that analysis naive. Al-Qaeda’s leaders in Afghanistan were executed and the organisation wiped out. In the long years after, the comparative lack of a terrorist atrocity on anything like the scale of 9/11 appears, for some, to be proof of the pointlessness of the mission, rather than an indicator of its success.

Even so, and in this case even if the US response has been the right one, it is hard to avoid the inescapable suspicions over who is calling the shots. Why were the West’s "red lines" suddenly tested again? Why was the strength of an extraordinarily narcissistic president suddenly put to the test? And why, three days later, do Russia and Iran appear to be agitating for what they call a “real war”?