Sometimes another company may be infringing on your invention, but you cannot stop them because they are doing business entirely in a country – manufacturing, selling, distributing – where you do not have patent protection. If you have patent protection in the United States and the infringer happens to be doing business entirely in Spain, where you have not been granted patent protection, it will be hard for you to justifiably stop the infringement of a business that has all of its operations in Spain.
However, keep in mind that such a company might distribute to another company which is in the United States. There may be in place some agreements or assets in the United States and therefore there may be opportunities to sue them in a United States court to stop the infringement. Having said that, it is also important to note that you may still have difficulty stopping the infringement when that infringement happens in a country where you do not have patent protection. Patent protection by nature is particular to each country. You should therefore file patents in every country in which you want to secure your right and protect against infringement. However filing in many countries is often times prohibitively expensive and so it is advisable to file primarily file in the country in which you are based in order to ensure your position in your particular market.