Ukraine

Its worked in Kosovo/Serbia and Korea.

And yes I think the peacekeepers will be well armed. But their real value is basically tripwire forces. Putin couldn't attack without effectively declaring war on the countries that send peacekeepers

I would really like to have heard the alternatives, that didn't involve direct intervention of outside (probably mostly US) forces. The current trajectory was/is not favoring Ukraine at all
The problem is Ukraine has insisted on relying on their own population and some very limited assistance in terms of boots on the ground. They've received a ton of logistical support and equipment/weapons, but soldiers on the ground are almost entirely Ukrainians or volunteer support from other countries. I don't think they have any outside peacekeeping forces on or near the front lines that I'm aware of.

And I'm not sure NATO can put boots on the ground at this point. Without the blessing of the US, I don't think anything happens like that. And Ukraine isn't getting into NATO without US support.

What could maybe happen is the EU on their own could send "peacekeepers" but I don't think the US or Russia would see it that way.

Ukraine is in an untenable position without the US fully behind them. I hope the EU doesn't back down and holds our feet to the fire, but I'm not sure Trump will care.