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We decided not to negotiate peace with the invaders.
Literal
Invaders [with-と] peace-treaty [of-の] negotiation [object-を] not-do [thing-into-ことに] decided.
~ことにする 'decide to do ~' takes a clause + こと 'fact/thing' + に + する 'make it (so that).' In the negative form V-ないことにする it means 'decide not to do ~.' インベーダー is a katakana English loanword, typical of sci-fi or game contexts — compare the classic Space Invaders / スペースインベーダー. 和議 'peace treaty, peace negotiations' is a formal, slightly literary word.