War on Terror:

The Taliban regrouped in western Pakistan and began to unleash an insurgent-style offensive against Coalition forces in late 2002. Throughout southern and eastern Afghanistan, firefights broke out between the surging Taliban and Coalition forces. Coalition forces responded with a series of military offensives and an increase in the amount of troops in Afghanistan. In February 2010, Coalition forces launched Operation Moshtarak in southern Afghanistan along with other military offensives in the hopes that they would destroy the Taliban insurgency once and for all. Peace talks are also underway between Taliban affiliated fighters and Coalition forces. In September 2014, Afghanistan and the United States signed a security agreement, which permits United States and NATO forces to remain in Afghanistan until at least 2024. The United States and other NATO and non-NATO forces are planning to withdraw; with the Taliban claiming it has defeated the United States and NATO, and the Obama Administration viewing it as a victory. In December 2014, ISAF encasing its colors, and Resolute Support began as the NATO operation in Afghanistan. Continued United States operations within Afghanistan will continue under the name "Operation Freedom's Sentinel".

Please answer a question about this article. If the question is unanswerable, say "unanswerable". What did NATO and the US sign in 2014?
unanswerable