Whatever you think of Colin Powell, I believe he had the Powell Doctrine right. It was also something he admitted he failed at regarding invading Iraq in 2003.
So, regarding Ukraine:
1. Is a vital national security interest threatened?
2. Do we have a clear attainable objective?
3. Have the risks and costs been fully and frankly analyzed?
4. Have all other nonviolent policy means been fully exhausted?
5. Is there a plausible exit strategy to avoid endless entanglement?
6. Have the consequences of our action been fully considered?
7. Is the action supported by the American people?
8. Do we have genuine broad international support?
The Powell Doctrine was responsible for ending Gulf War 1 before its objectives were met. They let Saddam go. They had to do Gulf War 2 just to catch the guy, almost all because of that dufus Powell.