I think you are very deeply mistaken. In 2014, when the annexation of Ukrainian Crimea took place, there were only 6,000 combat-ready troops in Ukraine, because Ukraine did not feel any threats and itself abandoned the world's third largest nuclear weapons potential. Therefore, the Crimea was captured almost without a single shot. In addition, Kremlin agents were in key positions in the army and law enforcement agencies, who gave the order not to shoot in any case.
Now the situation has changed dramatically. Ukraine has armed forces that occupy the 24th position in the world. Five Russian combat aircraft and a helicopter have already been shot down, two tanks and other military equipment have been destroyed. Putin will get a good rebuff and this will be the beginning of his end.
I didn't give any opinion about Ukraine's military, strength or resistance against the invasion for it to be right or wrong. I said US (or anyone else for that matter) is never going to get involved in this war to lead to WWIII (the title of this topic). You think US is going to risk world wide Nuclear Armageddon for Ukraine? No offense but they were just using Ukraine for their own benefit like a milking cow and abandoned them when the milk dried. Every US asset was evacuated from the region way before the invasion began. If they wanted to help they would have increased their forces instead of completely evacuation of the region.