Ukraine gave up its nuclear arms on the guarantee of security from Russia. Russia has violated that guarantee by invading them.
Putin can end the war today.
US can not end the war today. Even if it left Ukraine alone, Ukraine would still fight to the end.
EU can not end the war today. See above.
The fastest and easiest way to end the bloodshed is for Russia to withdraw today.
Leaving Ukraine to defend itself wouldn't even end the bloodshed. After Russia has completed its revised objectives it would invade the entirety of Ukraine under newer revised objectives. Then it would invade other neighbours under other revised objectives. The bloodshed would continue until the USSR is reformed and a new cold war begins.
Those suggesting the only way to end the bloodshed is through capitulation to the aggressor need to study their history better to see that capitulation to the aggressor never stopped an aggressor, it just lead to them going further until stopped.
Imagine if the USA invaded Mexico - no one would be saying Mexico should end the bloodshed. And the USA's adversaries sure as hell would be doing everything they can to help Mexico.
For those in the back: Putin withdrawing from his war of aggression is the only known way to end the bloodshed today. All other solutions would result in further bloodshed.