US is a first-to-invent country (at the moment) whereas Europe and most other jurisdictions have first-to-file rules.
That means that if a patent application was filed in the US and in Europe on July 1, 2004 and you found invalidating art with a publication date of June 1, 2004, that art would likely invalidate the European patent (I have not studied European law, but this is my understanding), but the US applicant would be able to avoid the art by declaring under oath that he/she/they had invented the claimed invention prior to June 1, 2004. This process is called “swearing behind” prior art.
It is fairly easy to swear back up to 6 months from a filing date to avoid prior art. More than that will require solid documentation. Beyond showing a date the invention was first conceived or reduced to practice, you need to show that you continued developing the invention (due diligence) up until the date the patent application was filed. As you can imagine, the longer you swear back, the harder it is to document first invention and continuous development.
The link below from the US Manual of Patent Examining Procedure explains swearing behind a reference.
USPTO
In Europe, it is possilbe to file an opposition against a granted patent (up to 9 months after the date of the publication of the grant). Opposition can be made on the grounds of non patentability (e.g. lack of novelty, lack of inventive activity), and/or absence of sufficient disclosure of the invention, and/or extension of the subject matter of the patent beyond the application as filed.
This is probably the “most efficient” way (time / money / substance – wise) to fight against a patent in Europe and obtain amendments or revocation of the contested patent.
