Why did the Incas prefer not to use their army to expand their empire

The Incas were great diplomats, and they were able to extend their influence throughout the Andes region by negotiating trade and tribute agreements, offering impressive gift exchanges, organizing inter-marriages, and relocating sympathetic populations to newly acquired or troublesome areas. Only when these strategies failed did warfare become necessary. Early Inca warfare was concerned merely with acquiring the wealth of the enemy but gradually, as they became more ambitious, they sought to permanently control the territory of their neighbors and so spread their influence across South America.