Software patents are a hell hole in the US and we want to get out of it by operating (at least initially) in Europe. If a US company only operates in Europe (say this is a web app) would we still be liable for patents in the US?
You need to consult a lawyer, period. Patent law is insane even when it is working well. Unless you are 100% certain the patent holder does not have a patent in Europe (and no one in Europe holds the same patent for Europe). But even then, the question you just asked, could be used to prove your intent to infringe. Don't eff around with legal issues, if your Idea is worth doing, it's worth doing right.
However... It is worth noting that it is the burden of the holder to enforce the patent, you can probably operate under the radar for a couple year s till you start turning coin from whatever it is you are doing, then you will get a cease and desist letter, unless you become an actual threat, then they will be coming to shut you down and take your money.
One final thing to note, the average north American patent rings in around $120,000 bare minimum. If a person can afford a patent, they can also afford to enforce it... the big question is will you be able to afford defending yourself?
yes, you can still be liable for patent infringement if you're based out of US, even if you do not sell in the US. The only way to be safe is to be a non-US company not selling in the US. US patent law can also stop foreign companies from selling in the US. There has been a bunch of patent lawsuits recently when a company XYZ is asking a gov. of a country ABC to block sales of a company X'Y'Z'. That's exactly whats happening.