The Brezhnev Doctrine asserted that the Soviet Union would start becoming an important and interfering force in the Eastern part of Europe for one reason only - to preserve communist rule. A part this doctrine was also that they would want to obtain a disarmament agreement between themselves and NATO.
The correct answer is the US senate, and president Wilson pushed in favour of the two. A possible reason for senate's rejection was a conflict with the president. Another possible reason was that the senators feared this would threaten the independence of the US.