Quit a PhD for another PhD During the last month, I applied for PhD programs in some of the top US Universities in my scientific field. At the same time, I applied also for a PhD position in a very good European University. About one month after I submitted my application to this European University, I was offered a full-time, funded PhD position, in which I was supposed to begin as soon as possible.

Admissions for the US Universities are announced in the spring. So, I currently have an offer from Europe and a very short deadline for accepting or rejecting this offer. My ultimate career goal, however, is to study and work in the US, and there are some serious reasons for this. If I reject this offer from Europe and if I won't be admitted in the US, I will have lost every chance for a PhD, since a long educational gap will have been accumulated, putting my career at risk. 

So, I'm thinking to accept this offer from Europe, until next academic year, and if I have a positive answer from the US, then quit my PhD studies in Europe and begin a new PhD in the US. I would like some insight about people in Academia. I acknowledge that such a scenario might affect the whole PhD project because of my possible withdrawal. Would this considered to be immoral? Could I have any consequences? Could this choice of mine - quit a PhD for another one, in a new country - seem bad to prospective supervisors in the US?

I already know that pursuing a PhD is a serious career choice, and I have no intention to take silly decisions for no reason. I just want the best for my career. To my eyes, this would be no different from quitting a job for a better one, for a better "company", in a "market" with greater opportunities. I would appreciate every advice from you people, since I have no experience related to Academia other than my undergraduate studies.
Quitting a PhD because you realise afterwards that it's not the right path for you is fine. A fellow student during my PhD did so (due to homesickness, primarily), and immediately started a PhD at a university in his home country. The supervisor/group leader/etc were disappointed but didn't hold it against him.

Quitting because you only accepted the position as an insurance policy is a very different matter. If it's not for personal reasons, such as those above, leaving mid-course for another university will be seen as a very selfish act. Even more so if anybody finds out that you were hoping to do it before you even started. From your description it sounds like your field is relatively small. That means people tend to know each other, which means this could very easily hurt your reputation far more than attending MIT (or whichever other prestigious institute) would help it.

In short: you shouldn't accept a position unless you intend to stick with it. You have an offer on the table, you can either accept it or reject it. If you accept it, the right thing to do is to withdraw any other applications which haven't yet responded. If you aren't willing to commit to it, the right thing to do is to reject the offer. There is a third option, too: you can ask the European university if they'd be willing to grant you an extension to the decision deadline. How likely this request is to be accepted strongly depends on many things, including national and institutional cultures and the length of deferral you're asking for.