The Landsarmy came into being by accident. As Salme Dien fled the fall of Tark, he was joined by a small group of similar refugees, grouping together for security. But, as a bully tried to force his way into power, Salma's training as a Prince Minor in the Commonweal led him to oppose that, and win the loyalty of the refugees. Initially unwilling to lead, he found he couldn't avoid it as his group expanded, and was hounded by bandits and Wasps.
Eventually, the group of people of all kinden were spread out across the entire area between the wasp-held lands and the free lands, all answering to Salma's command. He was able to convince Sarn to look after his non-combatants temporarily, in exchange for officially fighting as that ant state wished.
His troops are nearly all irregulars but that doesn't mean they are untrained. He fields the strongest terrestrial mounted force in the Lowlands and his snapbowmen are some of the best. Salma's strategy is such that he makes the best use of the advantages of each of the different kinden at his command. And when he has so many to choose from, there are many advantages.
The army only really fought in two battles: the Lowlands Ambush of the Sixth and Malkan's Stand before Salma's death meant that the Landsarmy effectively ceased to exist. The survivors continue to exist as the troops of Princep Salmae.