EU leaders gathered on Sunday to make a formal offer on a Brexit package deal to Prime Minister Theresa May to ease Britain out of the bloc in March. After 18 months of negotiation, including some last-minute brinkmanship from Spain seeking guarantees of a role in the future of Gibraltar, the 27 national leaders will endorse terms for withdrawal and an outline of future EU-UK ties at a tightly choreographed summit that will conclude with a visit from May.