Amber Rudd has chosen to resign because she felt she could not defend herself against the charge that she took her eye off the immigration ball.
Her excuse, to herself, was that her priority - for obvious reasons - was combating terrorism and improving domestic security.
But of course all ministers are supposed to multi-task, and she knew that excuse would not fly in public.
So she decided to stand down this afternoon, even before the Guardian published a leaked letter from her to the PM from January 2017 - which seemingly showed that far from being unaware there are targets for the expulsion of illegal immigrants, she actually set such a target.
Letters exchanged between Amber Rudd and Theresa May
Theresa May's response to Amber Rudd's resignation
My sources tell me that there were other such official papers about targets knocking around. And therefore she decided to quit - because she felt that MPs would simply never give her the benefit of the doubt.
In a way she has been hung out to dry by her own department.
It is extraordinary that her officials told her, before that fateful select committee hearing last week, that there were no targets for the removal of illegal immigrants.
And perhaps less extraordinary is that other officials leaked and briefed against her - since much of Whitehall is detached from ministers.
For the avoidance of doubt, she jumped: "I am told there was no pressure from the PM."
The point is that Rudd’s exit is arguably the most serious resignation May has suffered in her almost two years as PM.
For one thing the Home Secretary is one of the great offices of state.
More damagingly for May, the policy which underlay Rudd’s doom - the hostile environment for immigration which has caused so much unpleasantness for the Windrush migrants - was May’s not Rudd’s.
In other words, Rudd’s departure strips May of her human shield.