Alaska Air reached a roughly $1 billion deal to buy Hawaiian Airlines, a combination of two nearly century-old rivals that are dwarfed by the biggest players in a time of travel upheaval.

The proposed deal would unite carriers that largely serve destinations in the Pacific region and operate competing flights to Hawaii. Executives said the deal would solidify Alaska Airlines’ position as the fifth-largest U.S. carrier. But they will have to win over antitrust authorities, who have challenged recent airline tie-ups.