@TrPrado,
U.N. Security resolution 2118 was passed by the United Nations on September 27, 2013 which mandated that Syria, in cooperation with the United Nations, comply with U.N. chemical weapons protocol. The Russian Federation is a permanent member of the Security Council.
We know that prior to the chemical weapons incident on the 4th of April, the CIA had reported to the administration that military grade stockpiles were being housed at the Shayrat air base, an airbase in which Russian personnel are also stationed. After the incident on the 4th, the CIA reported to the administration that, in its opinion, the Syrian military was responsible for the attacks, which obviously both the Syrian and Russian governments denied to be the case.
The administration took the situation from there, ultimately reaching the point on the night of the 6th in which the Russian government was warned of an impending strike.