"Russian aggression against Ukraine is an attack on the agreed principles and rules of European and global order that protect global citizens beyond Ukraine, including Americans. Ukraine’s sovereignty and territorial integrity is protected by the UN Charter and European security agreements, which the Russian Federation has signed and is obligated to respect. The OSCE mission and Minsk agreement both obligate Russia to resolve the conflict peacefully with Ukraine. We must keep tough, targeted, and effective economic and financial sanctions on Russia as long as it continues to assault Ukrainian territory and citizens, and continues to illegally occupy Ukrainian territory in the Donbas and Crimea.
"But countering Russian aggression also means supporting Ukraine’s independence and ability to make and implement sovereign foreign policy decisions by supporting Ukraine’s political, economic, and defense capabilities. Although Ukraine is not a formal treaty ally, the U.S. should be willing to help Ukraine develop a modern and capable defense force to defend its citizens, including advice, education, training, and willingness to consider commercial sales of weapons appropriate to the situation. While the US must not exacerbate instability or conflict, we should not shy from responsible defense assistance to a democracy in the heart of Europe that is under assault because its citizens have chosen a democratic European path."
Source: 2019 https://www.cfr.org/article/pete-buttigieg