I am a finishing Ph.D student in pure mathematics in the US. In November, I received a 3 year postdoc offer in Europe on a PI's grant with a December reply deadline and accepted starting in 2015. Recently, I was offered an NSF MSPRF at an American school, starting in 2014. My plan is to use the NSF in 2014-2015 and 2016-2018 and go to Europe for 2015-2016--the NSF and my US host institution are OK with it. However, when I accepted the European offer I did not specify that I would be staying for only a year (I had no other offers at that point). A what point am I obligated to tell the European PI that I will stay for only a year? Now, before accepting the NSF? After starting the position in 2015? More generally, in Europe is it considered normal or unethical/breach of contract to leave multi year (mathematics) postdoc positions after a year? I know it's considered normal in the US but the postdoc hiring here is done at a departmental rather than individual level...
Going for one year would fulfill your obligations as I understand them, but I can't guarantee the PI will see things the same way. You should discuss it as soon as possible, to settle the issue and so the PI can at least make realistic plans. (I'd be a little offended if I discovered that someone visiting to work with me had dramatically changed their plans without telling me until much later, even if I thought the change was otherwise reasonable.)
There are several plausible outcomes:
The PI might insist that you have made a commitment to stay for more than a year. I don't think this is reasonable or likely, but it's better to find out now than later, so you can figure out what to do about it.
The PI might be fine with one year. Then you won't have to worry about this issue any further.
The PI might accept one year but seem unhappy about it. In that case, you could explain that you would really like to come for a year if possible, but you would be willing to withdraw if the PI would prefer to hire someone else for the full three years.
I think this depends a lot on the hiring practices from country to country and from university to university. For instance, if you're offered a multiyear position on what amounts to a renewable annual contract, then there's much less of a problem leaving after one year than if it's a single multiyear appointment.
However, essential communication with everyone involved is required before you start the positions. If people are not aware of what you want to do, and you spring it upon them as a surprise after you sign the hiring paperwork, you're setting yourself up for much bigger problems than if you talk to them and make sure everybody's OK with your plans in advance.
I think you know what is the appropriate action and what is not! You are probably hoping that somebody gives you a justification to do what you have planed. But even a teenager knows that when you accept to work with somebody for three years and later you change your mind and make another plan which conflicts with your agreement, you are obliged to let the PI know about your new plan as soon as possible.
Besides, there are probably several other good candidates to fill the position that you want to leave after one year. In this way you take an opportunity from a fellow human being without even using it!