Delta WAS responsible for rebooking your entire flight, even the international portion. The problem that is often run into, is that the international carrier, is sometimes not a U.S. Airline. If that is the case, it is more complicated to deal with those tickets, and sometimes it must be handled by that actual airline. In this case, if it was NWA that was your connecting international flight, Delta should have been able to assist you in having it rebooked.
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