Q: How to prevent a US copyright lawsuit after winning in Europe?

A: You might hope that the doctrine of res judicata would be helpful (https://jusmundi.com/en/document/publication/en-res-judicata). Sadly it is not necessarily recognized or applicable between different countries. Especially where you are dealing with explicitly national rights like patents and copyright.

You could perhaps seek a declaratory judgment of non infringement. That would involve filing your own lawsuit. It would be expensive and there could be problems establishing the court's jurisdiction over the other party.

A: Given that you've already successfully defended this issue in Europe, you should gather and preserve all documentation and court records from that case. This evidence can demonstrate clearly that your device does not infringe their copyright or patent, which may discourage them from pursuing litigation in the U.S. Additionally, consider having an attorney send them a formal letter that clearly explains your prior victory and that any similar lawsuit in the U.S. would likely have the same outcome.

If they continue making threats, you might consider filing for a declaratory judgment in a U.S. court. This proactive measure asks a court to rule officially that your device doesn't infringe upon their rights, effectively neutralizing their threats. Doing so could potentially save you from prolonged litigation later, especially if you intend to expand your business more aggressively in the American market.

Most importantly, maintain detailed records of their threats and communications, which could support your claim if litigation arises. Ensure your U.S. patent protections are fully up-to-date, clearly defined, and robust enough to support your position. Being proactive, transparent, and prepared will often discourage potential litigants who might reconsider if they realize you're willing to stand firm and defend your rights vigorously.