Co-authors decided to remove most of my contributions from a Nature paper without my consent I am a postdoc and have dedicated the past five years to working on a big project spanning several groups and countries.
My and my supervisor’s contribution was to interpret their results in light of our knowledge in my field.
For me, the project was an “aside” project, i.e. I have done volunteering work and the other authors explicitly said repeatedly that they were not the boss of this project.
We submitted a manuscript to Nature, with me being one of the first authors.
This article contained insightful information interesting for both our fields.

We got feedback from five different referees. Most of them saw our results as interesting but had many questions and one did not like it. The editor’s decision was that the manuscript could be considered again if the issues were addressed correctly. They did not request a change of format or length, respectively.

We have been rewriting the manuscript to make things clearer, and people from the other field have worked on their side for the past six months without giving me any updates. I discovered a couple of weeks ago that it is now a short letter and most of my contributions have been removed. I remain on the authors list.

When I asked why, no one answered. A rumor is that the part on my field was too complicated for them so they cut it because they believed that my contribution on their field is sufficient for submission. So in the end, the project started as multidisciplinary but it ended with a short letter addressed only to scientists of their field, not mine.

I disagree with this move since it was in my opinion illegal to take such an important decision without my consent and because the paper has lost a lot of interest for my publication list, now that a big part of my original work has been cut out.
Since I have worked on this for five years, this decision jeopardizes my career. My own director does not care but I do. 
The rest of the authors do not really care about my opinion and are saying that even without my consent, the paper will be quickly published with my name removed. I don’t think that is right.
They indeed asked me to write a new paper and publish it on my side, but I certainly won’t be able to publish it in the same journal (Nature) and the problem is that I finish my contract in a month. I guess it is somewhat possible to justify five years of work for a Nature paper, but for lower-ranked journal, it is more difficult.

If I decide to retract, can they publish the work as it is? Meaning that they still use my contribution to this work? If not, who and what department/lawyer can I turn to?
First, take the time to carefully consider your options. 

From your description the problem looks like a disagreement between co-authors: the behaviour of the corresponding author is careless and disrespectful, but it doesn't look like a major breach of ethics. Turning this into a legal battle might cause a lot of trouble, especially in a large multidisciplinary project. This could damage your reputation.

This is why I would suggest a more subtle approach first:


Ask your co-authors why the article has been shortened and why your parts have been removed.
Try to negotiate with them: explain why you think some parts you wrote are really important and should be added back
If this does not work or if too much of your work has to be removed, ask to use your contribution to write another paper for a different journal/conference. This time you would be the main author and present your work as you see fit. Your co-authors can hardly refuse this to you after cutting your part.