Interesting scenario! Here are a few possible outcomes that could happen:
1. The noble decides to remain neutral and not take part in the war, in order to maintain his oaths to both kings. This might be a difficult position to maintain, especially if one or both sides demand that he choose a side.
2. The noble tries to play both sides, offering his support to each king in turn. This could work for a while, but eventually, one or both sides may become suspicious of his loyalty and turn against him.
3. The noble chooses one side and breaks his oath to the other. This could have serious consequences, including losing his title and lands in one country, or even being hunted down as a traitor.
4. The noble attempts to negotiate a peace between the two countries, using his position and influence to broker a deal that would satisfy both sides. This would require a lot of diplomatic skill and might not be successful, but it would be a noble and honorable approach.
Ultimately, the decision the noble makes would depend on his character and values, as well as the political and military realities of the situation. It's an intriguing dilemma that could make for a fascinating story in a Dungeons and Dragons campaign.