A sharp disagreement over why Ukraine gave up its nuclear arsenal, and whether America broke a promise, cuts to the heart of the case for backing Kyiv.

• One side argues Russia would never have invaded if Ukraine had kept the nuclear weapons it inherited from the Soviet Union.
• The counterargument: Ukraine was too corrupt to be trusted with them and might have sold them to hostile buyers, which is why Washington pushed for their removal.
• That leads to the bigger fight — whether the US signed a treaty promising to defend Ukraine's borders, and whether its word means anything if it did.
• Both sides agree Ukraine's corruption is real; they split on whether that justifies walking away from the commitment.

Outlook: Expect this argument over broken guarantees to keep shaping the fight in Washington over how much more support Ukraine gets.