You have to go back further than that to understand CO's relationship in ST. In approximately 1998, CO had recovered from the 80's and early 90's. Posed on the brink of explosive growth CO had gone from likely bankruptcy candidate to potential player. NW and DL both saw this and decided the best course of action would be to acquire CO. CO was facing two hostile takeover bids. DL's unsuccessful bid would basically fold CO into the DL brand. NW's bid was a bit more complex, CO would exist as an independent carrier but would be bound to NW's management through the creation of the "Golden Share." The golden share gave NW basically veto power over major management decisions, alliances, mergers, ect.

Needless to say DL was not happy about losing the bid war to NW. This corporate bitterness as well as an overlapping route structure insured that DL would oppose CO at every turn, while NW would keep CO in ST.