Patenting an invention in the U.S. that may be patented elsewhere

First you need to understand what a patent does NOT do. A patent DOES NOT give you the right to make, use, or sell your invention. A US patent only gives you the right to exclude others--except the US Government--in the US and it's territories from making, using, and selling or importing your invention. An owner of an enforceable prior patent that is broader than yours and which reads on your invention can stop you, various laws can stop you, and the government can stop you from making, using, and selling your own invention. Your US patent has no effect outside the US or its territories and vice versa. Patents are territorial in nature. Only if the other person whose patent you want to patent in US has not filed any PCT with US being one of the designated countries, then you can very well patent the same in US otherwise not.