Mistake by the US not allowing Ukraine the freedom to hit military targets such as military bases or military staging fields. The reason is, this will prolong the conflict. Obviously the US should not condone free attacks on any target in Russia, but targeting bases should not be restricted.
If all Russian bases near the Ukraine border are destroyed, or at least heavily impacted, it will hinder Russia's ability to send troops into Ukraine. This has the effect of making it easier for Ukraine to be alerted to any troop movements if they have to come in from further away. Potentially saving Ukraine lives, and potentially helping to end the conflict sooner if Russia is forced to come to the negotiating table.
If all Russian bases near the Ukraine border are destroyed, or at least heavily impacted, it will hinder Russia's ability to send troops into Ukraine. This has the effect of making it easier for Ukraine to be alerted to any troop movements if they have to come in from further away. Potentially saving Ukraine lives, and potentially helping to end the conflict sooner if Russia is forced to come to the negotiating table.