The antonyms for the phrase 'were on one's side' are abundant, and a few examples include 'against', 'opposed', 'hostile', and 'unsympathetic'. Being against someone means to be opposing them, without offering support or assistance. If someone is opposed to a cause, idea or movement, then they are quite clearly not on the side of those supporting it. An individual can be called hostile if they are acting aggressively or maliciously toward someone, indicating that they are not on their side. Finally, someone being labeled as unsympathetic would imply that they are indifferent to the plight of the individual and that they do not care if they succeed or fail.
