Northwest has the right to veto agianst any alliance by KLM just as KLM has the right to veto against any alliance of Northwest for the comening X years (don't know the exact number of years. This is in their alliance agreement (at least according to their annual reports).
I can understand that. AF have an alliance with DL and now merging with KL there is 2 US airlines to deal with. I don't know how possible will be to have DL and NW as partners, that's now quite interesting!