While the maneuvering on Tuesday enabled Mrs. May to avoid a probable defeat in Parliament on Wednesday, what it means for the future of Brexit is less clear. A postponement of the Brexit date would require the consent of all 27 other European Union governments, and they are not likely to grant any extensive delay without a sound reason.

So unless Britain plans to hold a general election or another referendum, the delay is not likely to extend beyond early July, when a newly elected European Parliament is scheduled to meet.

In that sense, Mrs. May’s concession is probably less substantial than it seemed at first sight. Kenneth Clarke, a former Conservative chancellor of the Exchequer, warned that Mrs. May’s new guarantee amounted to no more than “giving us a date for a new cliff edge at the end of June.”

That thought may have occurred to some pro-Brexit hard-liners as well, whose reaction to Mrs. May’s shift was remarkably mild.

Since her Brexit plan was rejected last month by one of the greatest margins in British history, Mrs. May has put off one vote after another, pleading for time to renegotiate the deal to make it more acceptable. But critics accuse her of running down the clock in the hope that Parliament will vote for her unpopular deal as the only way to avoid the worse fate of a cliff-edge departure.