This week, Jeremy Corbyn suffered one of the largest backbench rebellions of his tenure, as about 100 Labour MPs failed to support a motion moved by shadow foreign secretary Emily Thornberry. Some, like Angela Rayner, were away for legitimate reasons. But scores of others apparently couldn’t bring themselves to support the leadership’s demand that Britain stop backing a brutal Saudi-led military campaign in Yemen. That was the point of principle on which they felt compelled to take this stand.

Although the UN places it in the same category of severity as the crisis in Syria, Yemen receives a good deal less coverage. In March 2015, a Saudi-led coalition intervened in Yemen’s civil war in support of the country’s recently deposed president. The move was backed by a deeply slanted UN security council resolution, which the Saudis and other regional monarchies took a leading role in drafting, and which was then rubber-stamped by their western allies. Leading experts on Yemen condemned the resolution, warning that what the country needed was national dialogue, not military escalation. Subsequent events have vindicated that judgment entirely.

Now 18 months on, much of Yemen lies in ruins. Schools, hospitals, homes and other civilian infrastructure have been bombed repeatedly by the coalition. At least 10,000 people have been killed, including about 4,000 civilians, mostly by coalition airstrikes. A UN report documented more than 100 strikes on civilian targets in the first nine months of the intervention, describing a pattern of such attacks that was “widespread and systematic”. The world’s leading human rights organisations and humanitarian NGOs all agree with that assessment. In testimony to a parliamentary select committee, one Unicef representative described “double-tap” strikes, where a second bomb would hit after the emergency services arrived. Foreign secretary Boris Johnson has condemned this tactic as “unquestionably a war crime” when referring to its use in Syria.

The cost of the blockade imposed on Yemen, the region’s poorest country, by the Saudis and their oil-rich friends, was dramatically illustrated on the front page of The Times on Friday, in a shocking photo of 18-year-old Saida Ahmad Baghili, whose body is so emaciated that one can scarcely believe she’s alive. Baghili lives near Hodeida, where a third of local infants suffer from acute malnutrition, and where residents were reduced to eating grass and drinking seawater after the coalition bombed Hodeida itself, Yemen’s major entry point for aid and food imports. According to the UN World Food Programme, 14 million Yemenis are going hungry, half of them now tipping into outright starvation, an outcome long predicted by aid agencies. UN officials report that the coalition often blocks or delays deliveries of even explicitly UN-approved food and medical supplies.

All sides in the conflict have been guilty of siege tactics and indiscriminate attacks on civilians, but the coalition is responsible for the vast majority of the suffering, and the coalition is the side that Britain is actively supporting. Indeed, Whitehall has approved £3.3bn of arms exports (including bombs and missiles) to Saudi Arabia since the intervention began, a huge rise on the equivalent preceding period. Calls to suspend those arms sales have been made by the UN secretary general, Save the Children, Amnesty International, and Human Rights Watch.

So you might think the Labour leadership’s demand that British support should be suspended, until the Saudis can be shown to be acting in accordance with international law and basic morality, would be an uncontroversial one. Apparently not.

Presenting the motion in the Commons, Thornberry was subjected to a series of ill-judged interruptions from Labour MPs such as Kevan Jones, Toby Perkins and John Woodcock. Indeed, Thornberry received more vocal support in the chamber from the SNP contingent than from her own supposed comrades. According to subsequent reports, some Labour members even tried to work with their Tory counterparts in order to defeat their own party’s motion.

Woodcock, a former chair of Progress, claimed that British support is “precisely focused on training Saudis” to improve their targeting, so as to “create fewer civilian casualties”, parroting the official government line. The idea that the Saudis’ “widespread and systematic” attacks on civilian targets are just a series of well-meaning errors is one that, to put it as gently as possible, lacks credibility. And if decades of training provided by the British to the Saudi pilots hasn’t prevented these supposed errors by now, it seems rather unlikely that it will in the near future.

In any case, this misrepresents the true nature of the British role. When the intervention began, then foreign secretary Philip Hammond pledged to “support the Saudis in every practical way short of engaging in combat”, including “spare parts, maintenance, technical advice, resupply” and “logistical support”. The reality is that the Saudi Air Force, roughly half UK-supplied and half US-supplied jets, could barely function without this ongoing assistance from Washington and London.

If there is Yemeni blood on the hands of the Saudi-led coalition, then that blood is also on the hands of the coalition’s western backers, enablers and apologists. The Saudis and their allies can only wage this war because the Anglo-American suppliers of their air forces are providing active, material support. And British and American politicians can only collude in these outrages because the political cost on them so far has been low. However, it is in the gift of their constituents – you, the reader – to change that equation.