For Russia, the US is not an assistant in the fight against terrorism but a bloody enemy if it supports terrorists in Syria.
A part of Ukraine is a Russian land that has never belonged to Ukraine and the people who live there do not have any connection to Ukraine.
Ukraine is recognised in international law as a sovereign nation state, with its own flag, nationality, language etc. Ukrainian citizens are citizens of Ukraine, not Russia. Russia has no right to claim them or to legislate for them.
The territorial sovereignty of Ukraine that includes the Ukrainian jurisdiction over Crimea also, has been recognized by the international community and enshrined in relevant UN resolutions.