It's at a summit that is her first Brexit test since an election sapped her authority.
Over coffee at the end of dinner on the first day of the EU summit, May will address the other 27 leaders and describe "principles" of her plan to give early guarantees to some three million people living in Britain who come from other countries in the bloc.
But her wings have been clipped - not only in Britain where voters denied her a majority in parliament, but also in Brussels where EU leaders will try to stop her from discussing Brexit beyond a quick presentation.
Instead, once she has left the room, they will continue their own discussion of Britain's departure from the European Union.
"My understanding all along is that this is a hugely important issue for Britain and for the 27 that has been clear from the very outset of this process," a senior British government source said of the question of EU expatriates.
"We want to provide early assurance, and it has always been our position that we want to outline our principles at this dinner and that is what we are going to do."
The source said Britain was "perfectly content" with the arrangements. Last week, one diplomat said May had tried to "hijack" the summit taking place on Thursday and Friday by drawing other leaders into wider discussions on Brexit.
Another British official said May would offer "new elements" in a paper to be published early next week. There may be sticking points with Brussels, such as the cut-off date for EU citizens in Britain to retain rights under the bloc's free movement rules.
To show the "goodwill" her aides often refer to, May will have a separate conversation with European Council President Donald Tusk and hopes to have other one-on-one meetings. But it is not clear whether she will make any headway on the Brexit talks, which began in Brussels on Monday.
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