Conservative whips were contacting backbenchers this weekend, warning them to be ready to lend their support to the prime minister as she makes the case for last weekend’s Syrian airstrikes.

Theresa May’s tendency to reserve judgment until she has studied all the evidence are sometimes a political handicap, serving her badly in the wake of the Grenfell Tower fire, for example, whose enormity she appeared slow to grasp.

But allies believe her step-by-step approach to decision-making and unshowy manner are cast in a different light when it comes to national security – particularly in the highly charged atmosphere that descends on the House of Commons when military action is discussed.

In the wake of the Salisbury nerve agent attack, May and her colleagues painstakingly and publicly built up the case for taking action against the Kremlin; a stance that won over most of the House of Commons – though not Jeremy Corbyn – and ultimately many of the UK’s international partners.

The prime minister will be keen to display the same kind of careful logic on Monday, as MPs return to the House of Commons after their fortnight-long Easter break to hear her case for joining the bombing raids in the early hours of Saturday morning.

Yet the stakes are far higher, and for several reasons. First and most obviously, sending diplomats back to Moscow is on a different moral scale to deploying bombers to attack Syria, putting lives at risk and intervening in a bloody and complex proxy conflict. Second, unlike in the Salisbury case, May failed to make the case to MPs – or, by extension, the public – before ordering action.

Downing Street claims that with parliament in recess, the timing was too tight, and insists anyway that in certain cases the government is free to order military action without prior approval.

Since the attack, it has published what it believes to be the legal case for intervention, and sent Boris Johnson out to answer some of MPs’ key concerns, making clear that the “timely, appropriate and commensurate” intervention is not meant to lead to a wider involvement in the Syrian conflict.

But a number of MPs, including party stalwarts such as Ken Clarke on her own side, were angered by May’s failure to consult them beforehand, adding to the sense that Downing Street was in too much of a hurry.

Donald Trump addresses the nation on the situation in Syria. Photograph: Mandel Ngan/AFP/Getty Images

And third, while the bombing raids were coordinated with key allies, and not unilateral, the driving force behind them was the US – and Donald Trump’s belligerent tweets throughout last week undermined the idea this was a carefully calibrated operation to maintain what May calls the “rules-based international order”.

This is a central idea in May’s thinking about foreign policy, much repeated in her speeches, including in Philadelphia last year, when she urged US Republicans to work with the UK to “recommit ourselves to the responsibility of leadership in the modern world”.

That speech – delivered en route to the infamous White House hand-holding trip – was intended partly to distance May from Trump, playing up the role of multilateral institutions that he had derided.

But it is hard to share the “responsibility of leadership” with a president so erratic and unbiddable – and while Corbyn is determined to highlight the risks of military action, and even question its legality, the prime minister faces a challenging start to the new parliamentary term.