For the last two years, Turkey and the U.S. have been at odds in regards to U.S. military assistance to the People's Protection Units (YPG), a branch and affiliate of the PKK, a recognized terrorist organization by the U.S. and the EU. When the U.S. administration decided to fight DAESH by using another terrorist organization by drawing an artificial border between the two and legitimizing the group, Turkey as a long-standing ally of the U.S. in NATO objected vehemently. The idea of trusting a terrorist organization to defeat another was not a rational, feasible and sustainable goal. The members of the same U.S. administration a few years before expected a similar result from fighting between al-Qaida and Hezbollah. According to The New York Times, some senior members of the administration "suggested that a fight in Syria between Hezbollah and al-Qaida would work to America's advantage." At least by then the U.S. was not assisting one of the sides.

But when the YPG and DAESH started to clash, the expectation of the administration was to support the YPG with military assistance so that it could defeat DAESH. And maybe they were expecting that the YPG, by then a U.S. partner, would become a legitimate group that could contribute to the peaceful resolution of disputes, ethnic harmony and stability in Syria. Or worse they may not have any "exit strategy" in helping the YPG. The opportunities and military assistance provided to the YPG were not provided to other opposition groups. Some in the West even romanticized the group and presented it like it was a mix between Robin Hood and John Rambo. Turkey was concerned because the group's ideological, organizational and personnel overlap with the PKK was neglected. As mentioned in several different instances in this column, as the PKK was killing Turkish citizens in city centers in Istanbul and Ankara and as it was ambushing Turkish security personnel with explosives, its Syrian branch was endorsed, trained and equipped by the U.S.The YPG's policies of demographic cleansing and its war crimes against the people in its captured areas are mostly ignored. The government's red line and concerns were "understood" by the U.S. but not acted upon. While U.S. advisers fought together with U.S.-backed and trained YPG members, the Turkish side was trying to demonstrate the dangers and risks of such an endeavor. According to Turkey, this was not a risky move for the national security of Turkey but for regional stability as a whole. However, it fell on deaf ears.

The romanticized YPG forces yesterday, under the air cover of the international coalition, attacked two Turkish tanks that were fighting DAESH in south Jarablus. One Turkish soldier died and three were wounded. In other words, the U.S.-backed YPG forces attacked the forces of U.S. allies. Although some tried to focus on the anti-tank missiles that were used by YPG members, that is only a detail. The real big news here is the YPG's decision to attack Turkey. Considering the PKK's attacks in the last two days in Cizre with dozens of tons of explosives that cost the lives of 11 police officers and wounded more than 70 and the rocket attack on Diyarbakır Airport, this only reaffirms the solidarity and similarity of the PKK and YPG. But for those who insisted on the artificial distinction between these two terrorist groups, this should be a game changer. A NATO ally and member of the international coalition fighting DAESH were attacked by the U.S.-backed YPG.

So what does this attack mean? This attack was a declaration of war on Turkey from the YPG. Since this attack took place after the recent public warning from U.S. Vice President Joe Biden to the YPG, it should also be considered as a challenge to the YPG from the international coalition. The international coalition needs to act decisively and consider different forms of sanctioning of the YPG. The empty threats or denunciations or meaningless warnings are not acceptable for Turkey anymore. The response to this attack should be powerful and decisive enough to deter any other attacks from the YPG. It should demonstrate the commitment of the members of the international coalition to protect their partners in the fight against DAESH in case of an attack from another terrorist organization in the region and the determination of the backers of the YPG to withdraw and if possible destroy all forms of military assistance. In the absence of such forceful action against the YPG, the countries and actors that are willing to fight against terrorist groups will reconsider because of the risk of being abandoned by their partners when they need help.

This attack also should be a lesson to the U.S. administration. For future reference, the risk of using one terrorist organization against another terrorist organization needs to be understood and evaluated. Now in Syria, we have thousands of YPG members that are trained and equipped by U.S. forces and time for the U.S. to think about an exit strategy from this plan of using terrorists to defeat terrorists. The threat from the YPG today may only be directed at Turkey, but it may soon to affect other countries.