Since the Cold War ended, the U.S. has largely overlooked its European strategy, but now Russia’s actions in Syria and Ukraine have forced the Pentagon to rethink its playbook. U.S. Defense Secretary Ashton Carter said Wednesday he did not expect Russia to change its behavior anytime soon and called Russia a “destabilizing influence,” Defense One reported.

“We must write a new playbook, which includes preparing to counter new challenges like hybrid warfare and cyber, better integrating conventional and nuclear deterrence in Europe, as well as adjusting our posture and presence to adapt and respond to new challenges and threats,” said U.S. Defense Secretary Ashton Carter Wednesday.

Cyberattacks have become a new front between Russia and NATO with reports describing how attacks have become more aggressive in recent months. Attacks have hit a wide array of Western targets including the U.S. House of Representatives and the Polish stock market. Russia has called the allegations of attacks “unsubstantiated.”

Carter also said that the Pentagon’s new playbook would need to focus on “mobility and highly ready forces that can respond very quickly.” When Russia annexed the Crimean peninsula in March 2014, highly armed, unmarked soldiers appeared on the streets and were nicknamed “little green men.” Russian President Vladimir Putin later admitted the little green men were in fact Russian soldiers.

“We will take all necessary steps to deter Russia’s malign and destabilizing influence, coercion, and aggression,” Carter said. “This is the new reality for us strategically. It looks like it’s here to stay.”

The U.S. has estimated that Russia has 25,000 troops in Crimea as well as ones in Eastern Ukraine where a conflict has taken the lives of over 8,000 people. The Russian government has continuously denied having any troops in the Donbass region of Ukraine.

Russia began airstrikes in Syria at the end of September. The U.S. and Russia are at odds over the war in Syria with the U.S. pushing for the removal of Syrian President Bashar Assad who has remained an ally of the Kremlin.

Carter said the Pentagon would not cooperate with Russia if the Kremlin continued to pursue its current policy in Syria.