The tensions around Ukraine; Russia publicly demands that Westerners renounce their military expansionism in Eastern Europe.
In fact, Russia is saying loud and clear that its security is threatened by 30 years of enlargements of NATO, the powerful military alliance between Americans and Europeans born after the Cold War and the confrontation with the USSR. Moscow is making Ukraine's possible membership a red line not to be crossed and is demanding that Westerners give up militarily support to Ukrainian forces which, since 2014, have been fighting pro-Russian separatists in eastern Donbass from the country.
On the other side, for the last 20 years, Russia has witnessed the entry into NATO of 14 countries which, to varying degrees, have been in its sphere of influence for decades. During these years, Moscow also watched helplessly as the Westerners decided to intervene in Kosovo in 1999.
More information on the history of the conflict between Ukraine and Russia, the current situation and the position of the western countries in the conflict can be found on attached links.