If one of the two group is not ready to establish peace, the neutral Muslims should support the peace seekers till the other group is also ready to give up oppression.Once the peace is maintained their disputes should be settled with justice.
In this fight one may be declared as an oppressor and other may be declared as just in the light of arguments and analysis but this fight does not exclude any one from Islam.
The fight between Sahabas of Rasool salal laho aliehi wasalam in Jamal and Safeen does not make any one non believer and does not break the bound of brotherhood between sahabas. This fact has been narrated in Quran in the following manner:
And if two parties or groups among the believers fall to fighting, then make peace between them both, but if one of them rebels against the other, then fight you (all) against the one that which rebels till it complies with the Command of Allâh; then if it complies, then make reconciliation between them justly, and be equitable. Verily! Allâh loves those who are equitable. The believers are nothing else than brothers (in Islamic religion). So make reconciliation between your brothers, and fear Allâh, that you may receive mercy. (Al-Hujurat 49:9-10)
These verses were revealed before the accident of Jamal and Safeen so we should look into these matters in light of Quran. May Allah give us wisdom to follow the right path Ameen.