Taking advantage of the new low in U.S.-Turkish relations, Putin saw his chance to use an S-400 sale to Turkey, so in July 2017, he offered the air defense system to Turkey. In the months that followed, the United States warned Turkey that a S-400 purchase jeopardized Turkey's F-35 purchase. Integration of the Russian system into the NATO air defense net was also out of the question. Administration officials, including Mark Esper, warned that Turkey had to choose between the S-400 and the F-35. That they couldn't have both.