Ever since Delta began ramping up operations in Seattle, Alaska Airlines’ hometown and main flight hub, the relationship between the two airlines has been disintegrating. And there was plenty to disintegrate. The carriers were long-time partners in each other’s frequent-flyer programs, and they code-shared on a host of flights. They were, in the industry vernacular, preferred marketing partners.

Against that backdrop of increasingly tense competition, this week’s announcement of a parting of the ways comes as no surprise:

Effective May 1, 2017, Delta and Alaska will no longer have a codeshare agreement, and sales of flights under DL- and AS-coded flight numbers flown on Alaska and Delta aircraft respectively will cease. Additionally, customers of each airline will no longer be able to earn and redeem Delta miles or Mileage Plan miles on flights operated by the other carrier. The carriers will retain an interline agreement, allowing them to continue offering customers ticketing and baggage connectivity. Delta and Alaska today codeshare on only a small number of flights as Delta’s growth in Seattle has reduced the need for codeshare flying.

So, as of May 2017, no more code-sharing, and no more reciprocal frequent-flyer earning and redemption.

The unraveling of the Delta-Alaska relationship is a timely reminder that such relationships are always provisional, subject to termination at any time. In this case, consumers at least have just over four months notice of the change, allowing them ample time to make any necessary changes to their flying or mileage-earning plans.

As for the effects of the relationship’s undoing, Delta loyalists are more likely to be negatively impacted than Alaska customers.

There’s no easy way for Delta to replace the earning and redemption opportunities afforded by Alaska’s west coast flight network.

On the other hand, Alaska’s acquisition of Virgin America will expand the universe of mileage-earning and -redemption opportunities, at least somewhat mitigating the loss of Delta. And with its mileage-based scheme, Mileage Plan remains one of the most valuable for average travelers.

Now, with the proverbial gloves off, the battle for Seattle, and beyond, will intensify. Which should be good for customers of both airlines.

Reader Reality Check

Who’s the biggest loser, Alaska or Delta?

After 20 years working in the travel industry, and almost that long writing about it, Tim Winship knows a thing or two about travel. Follow him on Twitter @twinship.

This article first appeared on SmarterTravel.com, where Tim is Editor-at-Large.