Delta Air Lines and Alaska Airlines were once close partners. But their relationship has deteriorated in recent years, largely the result of a turf war that sprung out of Delta’s move to build up its own hub in Alaska Air’s hometown of Seattle.

Now, the carriers have confirmed on Monday what many thought had become inevitable. Delta and Alaska airlines will officially terminate their frequent-flier and codeshare partnerships at the end of April.

The two airlines revealed the news in nearly simultaneous announcements Monday morning, saying their codesharing and frequent-flier partnerships would end April 30. Each carrier spelled out on their websites how the change would affect their customers (Delta | Alaska Air).

The relationship between Delta and Alaska Air once had been among the closest between two U.S. airlines. Their codesharing deal had allowed the airlines to place their two-letter code and sell seats on some of each other’s flights. Alaska Air and Delta also had a frequent-flier partnership that allowed their frequent-flier members to earn and redeem miles on either carrier’s flights.

Those pacts helped Alaska Air to feed domestic passengers onto Delta’s overseas flights, and Delta to connect its international passengers to other U.S. destinations via Alaska Air's Seattle hub. That allowed Delta to grow its international operation from Seattle -- especially on routes across the Pacific. Eventually, however, Delta began building its own hub at Seattle, where Alaska Air has long been the city’s top carrier. Many of Delta’s new routes began to compete head-to-head with Alaska Air, including on some of Alaska Air’s bread-and-butter routes -- such as those between Seattle and California and Seattle and Alaska.

As the Delta-Alaska relationship grew frostier, one local media outlet went so far as to call the partners "frenemies."

In announcing the end of its tie-up with Delta, Alaska Air’s also touted several upgrades to its frequent-flier program, already one of the most flexible among U.S. airlines.

In that announcement, Alaska Air reminded customers that -- starting Monday (Dec. 19) -- they can now earn Alaska Air mileage for flights on merger partner Virgin America and vice versa. Alaska Air closed on its acquisition of California-based Virgin America just last week in a deal that will make Alaska Air one of the most dominant carriers on the West Coast.

Also in its announcement, Alaska Air revealed it would reduce the number of miles to as low as 5,000 miles for short-haul one-way award tickets. Starting award prices for longer trips also have been reduced, the airline said. Additionally, Alaska Air it increased the number of miles its frequent fliers earn on many tickets booked on its roster of partners, which includes British Airways, Cathay Pacific and Emirates, among others. And Alaska Air said it would make all of its elite-level frequent-fliers eligible for complimentary upgrades on mileage award tickets when flying on Alaska-operated flights.

As for Delta, it said in its statement that it would “continue its commitment of investment and growth in the Pacific Northwest following a decision by Delta and Alaska Airlines to end their partnership.”

But the Atlanta-based carrier also conveyed an amicable tone, calling “the decision is a positive milestone for both airlines as Alaska focuses on its merger integration with Virgin America and Delta focuses on creating more customer choice at its Seattle-Tacoma International Airport hub.”