Why did the Biden administration first give Poland a "green light" to supply Polish "MiGs" to Ukraine but then change its mind?

ANSWER: Because the Biden Administration wanted the MiGs to be flown to Ukraine from Poland, not from a US airbase in Germany, which is how the Poles wanted to transfer the fighter jets. Why did the Poles want to transfer the jets from a US airbase instead of a Polish airbase? Because they did not trust the United States and other NATO member countries to come to their defense if they were attacked by Russia for giving the fighter jets to Ukraine. Putin has threatened war on any country that supplies Ukraine with fighter jets.  

Poland is a NATO member country, which means that it is supposedly under the protection of the North Atlantic Treaty, which says in Article 5 that an attack on any NATO member country "in Europe or North America shall be considered an attack on them all," and that means that the other NATO member countries would be expected to come to the defense of the NATO member country that is attacked. 

The fact that the Biden Administration changed its mind about giving Polish MiGs to Ukraine after Poland said that the fighter jets should be transferred from a US airbase in Germany instead of from an airbase in Poland suggests that the United States, and probably some other NATO member countries, would not have come to Poland's defense if it had been attacked by Russia, and that suggests that Article 5 in the North Atlantic Treaty is essentially just words on a piece of paper.

GREEN LIGHT

RED LIGHT