What would be the world wide reaction if starting tomorrow actual Gundams showed up and did an intervention in Ukraine against Russia to stop the war going on
Public perception outside of Russia would range from neutral to positive. Governments would ofcourse make the destruction of Celestial Being their highest priority. Don't want an uncontrollable organization that can show up with WMDs out of nowhere running around. It would be amusing if the mainstream media one day went from saying "Russia must be stopped! All who oppose Russia are heroes!" to saying "Celestial Being is dangerous because they stopped Russia!" the next day.
With such a demonstration of force, I would imagine that none of the superpowers would make overt military invasions, at least not until one country's military becomes confident in their ability to defeat Celestial being. Nations would continue to rely on low intensity methods to exert their influence over other countries. As low intensity conflicts drag on, people will begin questioning how much Celestial Being has truly helped, with some saying that by preventing overt wars, they are enabling these low intensity conflicts.
It is difficult to tell what is public opinion on the "special operation" inside Russia. Celestial Being may be viewed with resentment for killing sons whose only crime was being born in Russia and being conscripted and sent to a fight they may not have cared for.
The mecha industry would take off overnight, now that the world has seen that the world's most powerful battlefield weapons are mechas. Weapons manufacturers would be trying to figure out how Celestial Being made their Gundams.
what if it happened back in 2003 in Iraq against America?
Mixed reactions among the American populace. The country came together for a week after 9/11 happened, and that was it. The nuclear weapons cassus belli was already a meme from the start. A lot of people felt that the war was unjustified. However, people might view Celestial Being negatively if they feel that the US is being singled out and is not being allowed to defend itself, at a time when terrorist bombings in the US are happening left and right. Celestial Being might be able to mitigate this hit to their reputation if they also publicly destroy Osama Bin Laden and other such visible enemies.