The Islamic calendar does not count from the birth of the Prophet, nor from the first revelation, nor from any victory. It counts from the year in which Muhammad and his followers left Mecca for the oasis town of Yathrib, which became Medina, the city of the Prophet. The era was instituted under the caliph Umar some years afterwards, and the choice of starting point was a deliberate judgment about what had actually begun.
The migration followed years of pressure at Mecca, where the small community had been boycotted, harassed and in some cases driven to seek refuge in Ethiopia. The move north was arranged with representatives of Yathrib who had accepted the Prophet's message and invited him to arbitrate among the town's feuding groups, a role that made him a public authority rather than the leader of a persecuted sect.
What the choice of era says is that Islam dates itself from the point at which its community became responsible for its own affairs. Before the Hijra there is preaching, revelation and a group of believers. After it there is a polity: agreements binding the tribes of the city, a mosque as a public building, a treasury, a body of law developing case by case, and armed conflict with Mecca. The distinction is built into every date a Muslim writes.