President Joe Biden said Wednesday that Russian President Vladimir Putin made a "big mistake” by suspending his country's participation in the last remaining U.S.-Russia nuclear arms control treaty. The U.S. president was in Poland to reassure eastern flank NATO allies that the U.S. will remain by their sides amid the grinding Russian invasion of Ukraine. In his first comments since Putin's announcement Tuesday, Biden condemned the Russian decision to pull back from the treaty, known as New START. The decision to suspend Russian cooperation with the treaty's nuclear warhead and missile inspections follows Moscow's cancellation late last year of talks that had been intended to salvage an agreement that both sides have accused the other of violating.