The most accurate response to the question of how Old World diseases impacted warfare in the New World is:
Diseases substantially weakened Indigenous armies.
Old World diseases such as smallpox, measles, and influenza had a devastating effect on Indigenous populations, who had no prior exposure or immunity to these illnesses. This led to significant population declines, which weakened Indigenous societies and their ability to resist European conquest and warfare effectively. The demographic collapse due to disease significantly impaired the military capabilities of Indigenous armies.