Why did Theresa May capitulate to the ERG Brexiter rebels, imperiling her Brexit plan?
And why did she do so when even in her own party the Brexiter ultras are a small minority, and an even tinier minority in parliament as a whole?
It is because the Brexiter ultras - as opposed to the Remainer ultras - are fundamentalists; when push comes to potential Brexit betrayal, they rate an attempted redemption of the Brexit dream above loyalty to party.
Or to put It another way, if they thought it would keep the Brexit show on the road, they would even make what to them would be unholy common cause with Labour; they would even bring down the government.
So they are much scarier to May and her whips than the Morgans and Soubrys of the Remainer vanguard.
Monday evening’s votes showed that even when the Remainers see the PM wrecking her own Brexit proposal by kowtowing to the Brexiters, they still can’t muster more than 14 to see off the putative enemy within.
So May won and lost on Monday.
She made the right judgement by siding with the Brexiters, because she won her Customs Bill votes - by the narrowest of margins!
But she also lost, because she has been taken hostage by the Brexiters - and she won’t be released unless and until she repudiates the collecting of tariffs for the EU (which the Brexiters believe she did tonight) and the following of EU rules for the production and consumption of goods and food.
What cripples May is that although there are many more Remainers than Brexiters in Parliament, only the Brexiters will take a bullet for their cause.