Article 4 was invoked by Turkey in 2015 in response to developments in Syria and by Poland in 2014 in response to developments in Ukraine (Economist, NATO, Radio Free Europe/Radio Liberty). Article 5 has been invoked once, by the United States in response to a terrorist attack in 2001 (NATO). _{Confused? Check our FAQ or ask us for help.}

---

_{If you liked this question, you might also enjoy forecasting on whether India will sign either of the two remaining defense foundational agreements with the U.S. before 2018 or on whether the U.S. provide notice of intent to withdraw from NAFTA before 30 April 2017.}"