I don't think US power is slipping. We still spend an insane amount of money to keep troops stationed around the world, and this has two effects. One, it means the US has troops just about everywhere. Two, many countries have relatively small military budgets and armed forces of their own, which would change drastically if the US took its ball and went home. US decline is, essentially, world decline.
Ukraine is in a vital position between Europe and Russia. It is a reliable point for launching a war in either direction, providing cover for the army holding it, while forcing the other side to funnel its forces. It also has a lot of fertile farmland and considerable oil and natural gas reserves. NATO would love to have Ukraine join it, as it would allow them to seriously reduce the threat from Russia. Russia would love to have it for exactly the opposite reason-- it would provide them a much more secure front and allow them to keep some pressure on NATO.
The problem for Russia is that the invasion has dragged on for far too long and cost them significantly. For the west, the invasion has worked out very well-- they are able to wear down Russia both militarily and economically, while leaving the fighting --and dying-- to Ukraine. I'm not sure there is a way out for Russia at this point. Even if they win, the cost of rebuilding Ukraine without western support would be overwhelming. And yet, a defeat or withdrawal would also be bad economically, as well as a political disaster.
I don't think it will lead to WW3. For the west, there is no reason to get any more involved than they already are, sending massive amounts of money and materiel to Ukraine and letting Russia wear itself out. For Russia, they are in a trap of their own making, and have no real option other than to keep going until they defeat Ukraine or run out of men and weapons. Perhaps, if Putin is removed, they will be able to end the war and put the blame on him, but it would still be a disaster for them.
I expect that politicians here will continue to gripe openly about it, but behind closed doors they are quite content to keep sending aid to Ukraine. Putin had hoped to restore some of the glory of the former Soviet Union and perhaps begin to rebuild the buffer between it and Europe. But that has backfired and the worst may still be to come.