There is a backstory concerning the MIG 29s Poland offered to give to Ukraine. It would have been a simple matter for Ukraine pilots who are checked out on MIGs to come to Poland and fly them home. But that wasn't what Poland offered.
Poland offered to fly the planes to a US air base in Germany, where the Ukraine pilots could pick them up and fly them home. Why the additional step, you may well ask?
Poland was concerned that Russia would view their gift as an act of war against Russia and an excuse to attack Poland. Their offer to hand off the planes to the US who would then give them to Ukraine ensured that the US was as much guilty of the transfer as Poland, and that would be sufficient to deter the Russians from invading Poland.
The US isn't willing to go to war with Russia over its attack on Ukraine, a non-member of NATO. So we nixed the MIG deal, on the off chance Russia would include us in their retaliatory attack. It is also possible that the US wasn't excited about replacing Poland's old MIGs with new F-16s as Poland had asked.
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Another factor might be the offensive capabilities of the MIG 29, it has a 770 nautical mile range which equals roughly 1400 kilometers. The distance from Kyiv to Moscow is just over 800 km per Google Maps.
Ukraine could choose to use their Polish MIGs to bomb Moscow in a sort of latter-day Doolittle nuisance raid, certainly justified from Kyiv's viewpoint. If that happened, who knows what Putin would do? Would he nuke Kyiv? He'd have to respond with dramatic force, when we'd like to see less escalation, not more.
