On 17 March, United Nations Security Council Resolution 1973 was adopted, authorising a no-fly zone over Libya, and "all necessary measures" to protect civilians. Two days later, France, the United States and the United Kingdom intervened in Libya with a bombing campaign against pro-Gaddafi forces. A coalition of 27 states from Europe and the Middle East soon joined the intervention. The forces were driven back from the outskirts of Benghazi, and the rebels mounted an offensive, capturing scores of towns across the coast of Libya. The offensive stalled however, and a counter-offensive by the government retook most of the towns, until a stalemate was formed between Brega and Ajdabiya, the former being held by the government and the latter in the hands of the rebels. Focus then shifted to the west of the country, where bitter fighting continued. After a three-month-long battle, a loyalist siege of rebel-held Misrata, the third largest city in Libya, was broken in large part due to coalition air strikes. The four major fronts of combat were generally considered to be the Nafusa Mountains, the Tripolitanian coast, the Gulf of Sidra, and the southern Libyan Desert.