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
Not everything that glitters is gold.
An old friend is better than two new ones.
Оne can not see the wood for the trees.
Silent means consent.
Where there is no fish even a crayfish is a fish.
Poor people are inventive.
Your own shirt is closer to your body.
Some to the forest, some for firewood.