Learn German: Die Eindringlinge werden unsere Feinde. - The intruders are becoming our enemies.

Nouns ending in the suffix -ling refer to people, animals, or things with specific traits, and they are always masculine (der Eindringling). To form their plural, they reliably take an -e without an umlaut, making it die Eindringlinge.

What verb does the noun Eindringling come from?

It comes from the separable verb eindringen, which means to force one's way in, or to intrude. Adding the -ling suffix to a verb stem is a common way to describe a person who performs that action.

Why is unsere Feinde in the nominative case instead of the accusative?

Because of the verb werden. Just like sein (to be) and bleiben (to stay), werden (to become) is a linking verb. It equates the subject with a description or identity, so both parts of the equation must take the nominative case.

Doesn't werden usually indicate the future tense? Why is it translated as "are becoming" here?

Werden is used as a helping verb to form the future tense, but only when it is paired with another verb in the infinitive at the end of the sentence. When werden is the only verb in the clause, as it is here, it acts as a main verb meaning "to become" or "to get."