The National Interest looks beyond Article 5 of the 1949 Washington Treaty (the attack on one attack on all treaty):
The treaty Turkey, a NATO member, signed the Baghdad Pact in 1955 to become a member of CENTO. In carrying out its obligations under CENTO, Turkey could not expect that other NATO members would automatically come to its aid. (Echoes of this were sounded in the run-up to the 2003 Iraq War when Turkey asked for consultations under Article 4 of the Washington Treaty. Some NATO members took the position that if Turkey were attacked by Iraq after supporting the U.S. invasion and allowing its territory to be used as a jumping-off point for U.S. forces, it could not expect automatic support under Article 5.)
In short, Article 8 was designed to prevent individual NATO member-states from offering actual or implied security guarantees to other, non-NATO members and therefore expecting that NATO as a whole would be bound to honor those guarantee.