Encyclopedia of the Incas

The chronicler Juan de Betanzos describes generational campaigns in which
the mobilization of the army coincided with the construction of infrastructure
that facilitated future military incursions into a conquered region. The
construction of roads and bridges enhanced the reach of Inca power, and
Betanzos notes how the construction of the strategic bridge over the Apurímac
River during an early campaign led immediately to the surrender of multiple
groups living on the other side of that natural barrier. Betanzos and other
chronicles describe Inca conquest ideology in the early years as motivated by
respect for the ruler’s title and for the Sun cult.
Imperial campaigns in the early fifteenth century involved universal
conscription of an overwhelming force of Inca subjects, a peasant army under
the direct strategic direction of the ruler and a few of his closest male relatives
(see Warfare). Over time, the growing empire found it useful to raise troops
more frequently, and by the middle of the fifteenth century it was not uncommon
for multiple armies to be in the field at the same time, led by two or three
brothers or sons of the ruler. Delegation of command allowed the Incas to
expand along the axis of the Andean highlands while also making forays toward
the Amazonian slope to the east and the Pacific coast to the west.
To a certain extent, rulers seem to have delegated command as they grew older
and shifted their focus to ritual and administrative power seated at the capital.
Vesting one’s sons with military power was seen as a way of allowing potential
future rulers to distinguish themselves, and to channel some of the negative
effects of sibling rivalry from the capital into unconquered parts of the Andes.
Some Inca princes perished on campaign, but those who returned victorious
brought prisoners and spoils to offer to their father so that he could celebrate his
victory—even when the ruler was so feeble that he had to be carried through the
ceremony.
The speed of early Inca campaigns relied on attracting allies using intimidation
as well as promises of marriage alliances and rich gifts for cooperative local
leaders. Over time, many Inca allies rebelled, and the chronicles clearly link
rebellion to periods of royal succession, which frequently came with factional
violence in the Inca imperial heartland (see Wars, Dynastic). At such times,
Inca military power was directed at itself, and the personal connections that local
leaders had forged with a deceased ruler were no longer certain. Many chronicles
identify the accession of Topa Inca Yupanqui as a time of pronounced rebellions
that required costly campaigns of reconquest.