Undoubtedly all the sabre rattling out of Washington this past month was being coordinated with this move by India. Although Karzai might be chafing under the US bridle, the Indian foreign policy establishment can’t seem to do anything really contrary to US wishes, so I doubt this was done without Uncle Sam’s blessing.
Looks like the squeeze is on for Pakistan to accept a US-India security umbrella or else. The alternative is an outright and somewhat hostile move to an exclusive alliance with China, which would cost the nonmilitary elite dearly in various cultural terms. Let’s see if they have the inclination, or if the rifts inside Pakistan’s polity worsen.
From the Indo-US perspective, this strategy has the merit of being multifunctional — good vis a vis China and also good if Pakistan breaks up.