A United States patent is only valid within the United States and will not neccesarily allow you to sue infringers in another country. Most countries have their own patent system, and if you plan on operating internationally you need to consider international protection. While there are many different patent systems, the requirements are generally the same: your invention must be new, useful, and non-obvious. Some jurisdictions may even require you to seek patent protection prior to publicly disclosing an invention- which differs from United States law, wherein you have a period of time from public disclosure to apply.
One of the easiest ways to begin pursing international patent protection is through a Patent Cooperation Treaty (PCT) filing. The PCT allows you to file an international application with the World Intellectual Property Organization (WIPO). From WIPO, your application can be submitted to any of the member nations, which includes most major countries around the globe. This process is referred to as entering the national stage, and once you enter the national stage you must follow that nation’s requirements to obtain a national patent.
Unfortunately, the PCT application does not lead to an “international patent.” However, the PCT application serves as a global provisional application, and you have over two years from filing the PCT application to enter the national stage in as many countries as you wish to obtain protection in. A PCT application is often more efficient than trying to coordinate filing in multiple countries.