Actually there were plenty of opportunities for both Europe and the US to stop this war from happening. First they shouldn't have invited Ukraine to join Nato in 2008, and instead respect its independence. Russia has warned the West many times, that this invitation is a red line for her, yet Russians did nothing back then. They trusted that the thing can be settled peacefully. Then when the democratically elected president of Ukraine officially backed off from joining the EU and NATO in late 2013 early 2014, he was immedietly thrown out of power by ultranationalist elements - with the active support and in the presence (!) of US politicians on the ground. A civil war broke out, Crimea asked for support from Russia and wanted to join her to avoid becoming a sloughterhouse. Then came the Donbas, but its leave was prevented by joint Nato and Ukrainian forces. The following peace agreement (Minsk I and II) was later turned out to be a fake (admitted later by both the Ukrainians and the presidents of Germany and France) and was used to build up the biggest army in Europe after Turkey. This all could have been prevented. Russia did nothing to stop this. Then in the run up to the current events, this massive army has started to shell the breakaway regions and openly threatened to overrun them by force. This is where Putin stepped in and said no. The rest is history.