The Portuguese and the Spanish had a strong firepower and army. But along with these, germs and viruses helped them in conquering parts of America. As America was isolated earlier, the original inhabitants had no immunity against the dreadful germs and viruses present in Europe.
The diseases once introduced, spread very fast. The diseases, like smallpox decimated whole communities, paving the way for the conquest of the colonisers.
These germs were more dangerous as compared to guns and firearms because guns could be bought or captured, but there was no way to escape from these diseases.
Thus, the Portuguese and Spanish colonisers conquered the local population easily to enable colonisation of the Americas.