Given a travel related situation, and given a specific question about the situation, summarize the question into a title

When travelling from the US to an EU country, and involuntarily denied boarding, there are two different entities that could regulate the compensation:

The US Department of Transportation has rules based on hours delayed and percentage of the ticket.
The EU has rules based on kilometers traveled. In addition, it includes hotel costs and meals.

Which one of these rules would apply?
Bumped on transatlantic flight - compensation rules?