The Quran

وَإِن نَكَثوا أَيمانَهُم مِن بَعدِ عَهدِهِم وَطَعَنوا في دينِكُم فَقاتِلوا أَئِمَّةَ الكُفرِ ۙ إِنَّهُم لا أَيمانَ لَهُم لَعَلَّهُم يَنتَهونَ

Muhammad Asad

But if they break their solemn pledges after having concluded a covenant,¹ and revile your religion, then fight against these archetypes of faithlessness² who, behold, have no [regard for their own]pledges, so that they might desist [from aggression].

Lit., "if they break their oaths after their covenant". This obviously refers to unbelievers who, without having renounced their own beliefs, have concluded treaties of friendship with the Muslims. Their subsequent "breaking of the solemn pledges" is an allusion to the breach of the truce of Hudaybiyyah by the pagan Quraysh, which, in turn, led to the conquest of Mecca by the Muslims in the year 8 H.
The word imam (of which a'immah is the plural) denotes not merely a "leader" but also - and primarily - "a person who is an object of imitation by his followers" (Taj al-'Arus): hence, a "model", or "exemplar", or "archetype". The term kufr, which usually signifies a "denial of [or "refusal to acknowledge"] the truth", is rendered here as "faithlessness" because it refers, specifically, to a deliberate breaking of solemn engagements.