There were a lot of wars in which the USSR and the USA supported two opposite sides. The most famous of these wars is the Vietnam War.
Usually the USSR and the USA acted as follows - they supplied weapons to the parties to the conflict and sent military instructors. Including it could be pilots of military aircraft.
Of course, a large-scale war in the middle of Europe is a very big risk of further escalation of the conflict.
In particular, in my opinion, Poland is psychologically ready to take the side of Ukraine. In certain situations, it is probably ready to send its soldiers to participate in hostilities against Russia.
Since the time of Peter the Great (this is the end of the 17th, the beginning of the 18th centuries), in all the wars waged by the Russian Empire, and then the USSR, Ukrainians participated on their side, and this gave them additional power, since Ukrainians have always been distinguished by resourcefulness and the conduct of non-standard military actions and free spirit of Zaporizhsky Cossacks. Now, for the first time, Russia has launched a large-scale war against Ukraine at the state level, and it cannot and is not able to win this war, despite more than a tenfold superiority in manpower and equipment.
Some countries, such as Poland, may be ready to support Ukraine with their regular troops, but Ukraine has not yet asked for such help and, most likely, will manage on its own. True, in the ranks of the Armed Forces of Ukraine there are volunteers from various countries, mainly from the CIS countries, who, with the help of Ukraine's victory, want to overthrow their anti-people regimes.
The war between Russia and Ukraine is a disaster. Now, unfortunately, the closeness of these two peoples is denied, but for centuries they interacted well with each other.
There were a lot of mixed families in the country, where one spouse was Russian by nationality, and the other was Ukrainian. There are many citizens in Russia with Ukrainian surnames, and in Ukraine there are many citizens with Russian surnames.
In the USSR after Stalin, all the General Secretaries of the USSR came from the territory of Ukraine (Khrushchev, Brezhnev, Chernenko, Andropov, Gorbachev).
This is not surprising. The largest regional communist party in the USSR was the Ukrainian Communist Party. In fact, the Ukrainians were the political elite of the USSR.