Is a Master's degree in Europe valid for a teaching career in the USA?
The short answer is that your EU Master's degree would typically be about as useful as a US Master's degree in your subject area, which is not very helpful by itself.  

Most of the requirements to be licensed as a teacher in most states in the US have more to do with what training you've had in education rather than your subject matter expertise.   

A further important point is that if you aren't already a US citizen and unless you can immigrate to the US through some other pathway (e.g. by being married to a US citizen) you'll find that it is difficult if not impossible to get work authorization to teach in a school in the US.