Three months later royalist general Pío Tristán advanced in the north with more than three thousand men, prepared to invade the United Provinces. Once again outnumbered by larger armies, Belgrano organised a great exodus of the city of Jujuy: the entire population of the city would have to retreat with the army and not leave behind anything that might be of value to the royalists (such as animals, crops, or housing). By September a proper formation of columns provided them with a victory against a royalist task force of 500 men during the Battle of Las Piedras. The First Triumvirate commanded Belgrano to retreat to Cordoba without fighting, but he thought that doing so would mean the loss of the northern provinces. Thus, instead of continuing to Cordoba, he was convinced by the people of San Miguel de Tucumán to make a stand there. His forces had increased by then to nearly 1,800 soldiers, still much less than the 3,000 at Tristan's command. Even so, he obtained a victory in the Battle of Tucumán. By that time, the First Triumvirate was replaced by the Second Triumvirate, which provided greater support for Belgrano. The Second Triumvirate called the Assembly of Year XIII soon after taking power, which was intended to declare independence and enact a national constitution, but failed to do so because of political disputes between the members. It did not take measures regarding the national flag, but allowed Belgrano to use the blue and white flag as the flag of the Army of the North.