ally / competitor
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ally
A person who fights or works together with others for a goal, an idea, or a conviction; an ally or comrade.
Für die Umweltbewegung sucht sie noch engagierte Mitstreiter.
She is still looking for committed allies for the environmental movement.
The word is made up of the prefix 'mit-' (together/with) and 'Streiter' (fighter). A 'Mitstreiter' (ally/competitor) is therefore literally someone who fights alongside someone, either on someone's side or in competition with someone.
Er war einer der ersten Mitstreiter in der Bürgerbewegung.
He was one of the first allies in the citizens' movement.
The context tells you whether an ally or a competitor is meant. If it is about a shared idealistic goal, it is an ally. If it is about competition in business or sports, it is a competitor.
Für seine Kampagne gegen Plastikmüll sucht der Aktivist noch Mitstreiter.
The activist is still looking for allies for his campaign against plastic waste.
In the meaning 'Verbündeter' (ally), the preposition 'für' (for) is often used to name the shared goal. You look for or find 'Mitstreiter für' (allies for) an idea, a cause, or a movement.
Die Organisation fand viele Mitstreiter für den Schutz der lokalen Wälder.
The organization found many allies for the protection of the local forests.
soldier
'Soldat' and 'Mitstreiter' can both be used figuratively in connection with commitment to a cause. 'Mitstreiter' means a person who works or fights together with others for a goal. The word therefore emphasizes community and a shared goal. 'Soldat' literally means a military person; in a figurative sense, it tends to emphasize discipline, loyalty, and obedience. The words can be confused because both can stand for committed people in figurative expressions. If someone is working on the same side, 'Mitstreiter' is more natural. 'Soldat' fits better when you want to emphasize a very loyal and disciplined attitude.
Bei der Arbeit sucht das Team neue Mitstreiter für das soziale Projekt.
At work, the team is looking for new allies for the social project.
Herr Müller war für den Verein immer ein treuer Soldat.
Mr. Müller was always a loyal soldier for the club.