Naturalize in this context refers to an official legal process involving an application and approval. After meeting the residency requirement, he applied to naturalize.
Pro Tip 2/3
Not Instant
Becoming a naturalized citizen usually takes several years; it's not an instant change. She lived in the country for five years before she could naturalize.
Pro Tip 3/3
Ceremony
Often, naturalization includes a ceremony where new citizens swear allegiance to their new country. After naturalizing, he attended a ceremony to pledge loyalty to the flag.