With accent marks:
Оди́н в по́ле не во́ин.
One in the field is not a warrior.
This proverb is used to emphasize that only acting together people represent a force that can overcome difficulties.
However, there is also an opposite proverb in Russian:
И один в поле воин.
Even one is a warrior in the field.
This means that there are situations when a lot depends on one person.
In European languages, there are similar proverbs that speak of the limited abilities of an individual. The closes equivalent in English: there is safety in numbers.
Other Russian proverbs
When the job is done, you can go for a walk.
The apple never falls far from the tree.
Don’t wave your fists after the fight.
Eggs don't teach the chicken.
Fear has big eyes.
Not everything that glitters is gold.
Silent means consent.
All is well that ends well.