Under Putin, Russia's relationships with NATO and the U.S. have passed through several stages. When he first became president, relations were cautious, but after the &lt;a href="September%2011%20attacks"&gt;9/11 attacks&lt;/a&gt; Putin quickly supported the U.S. in the War on Terror and the opportunity for partnership appeared. According to Stephen F. Cohen, the U.S. "repaid by further expansion of NATO to Russia's borders and by unilateral withdrawal from the 1972 Anti-Ballistic Missile Treaty", but others pointed out the applications from new countries willing to join NATO was driven primarily by Russian's behavior in Chechnya, Transnitria, Abkhazia, Yanayev putsch as well as calls to restore USSR in its previous borders by prominent Russian politicians.