Without local tribes? Probably not, but that’s almost always how conquering actually happens (by exploiting existing fault lines). The situation with Alexander the Great is kind of representative. Alexander the Great had an army of about 30,000 people and conquered the Persian Empire which had a population of about 50 million. The Conquistadors had 3000 and conquered the Aztec Empire which had a population of 5 million, although the Conquistadors also had the benefit of disease traveling before them and not just better tactics but also far superior metallurgy. It doesn’t necessarily take an enormous advantage to conquer large territories, and the Conquistadors had numerous advantages.