Breaking off a collaboration/project in science I discussed with two fellow PhD students about working together. I have been working on my project for about 8 months, and we started working together around 3 weeks ago. In the beginning we agreed I would be the first author, since I have been doing most of the work and the project is mostly done. After starting the work, however, one of the PhD students has been continuously promoting his work in the project as having more contribution (e.g. he always mentions his work first and mentions mine in a side sentence), even though I have been working on the project for a lot longer than he has.

Recently, we had a visitor that could potentially help with our project. So we decided to give our work in three separate presentations. I said that I could give the introduction for our larger project and we agreed on that (after some very small unspoken disagreement on the mentioned person's part). During the actual presentation, however, my fellow student friend announced that he'll give the introduction (to which I agreed since we were in front of an audience).

So, after this I have a bad taste in the collaboration and would like to break it off without causing trouble. My supervisor and a lot of other people know we are supposed to work together now.

Any suggestions on what I could do? They did some initial work so I am ok with putting their names on my paper if it comes down to it, but I do not want to continue with the collaboration.
I assume that the two other students were not "forced" on your project by some higher authority, so the call on whether or not to collaborate is yours to make.

If that is the case, you should as soon as possible sit down with the other two students and tell them that you are not going to work with them any longer. The major point here is be civil and professional. If their contribution is large enough, you can put them as co-authors or in the acknowledgements section. I say as soon as possible, because 3 weeks is not that much of time for them to get too seriously involved. For the purpose of other people, all three of you should assume a neutral stand point, i.e. something along the lines of "things didn't work out" without going into extensive details. Although, you should definitely bring your adviser up-to-date the real reasons for stopping the collaboration. It is never good to wash your dirty laundry in public, so badmouthing on either side just hurts all the involved parties (one reason more to keep the adviser in the loop).

Be adamant about your decision, as I guess that your colleagues and the adviser will try to smooth things out one way or the other (asking you to reconsider, for some more time, increasing their contribution, etc).