Complex sentence: Foremost among these is the notion that the Partisans who attacked at the Via Rasella should have turned themselves in; this stems from a belief (still cultivated by neo-fascist propaganda) that the Nazis gave warning to the Roman public that a retaliation was imminent.
Split into simpler sentences: Foremost among these is the notion that the Partisans who attacked at the Via Rasella should have turned themselves in. This stems from a belief (still cultivated by neo-fascist propaganda) that the Nazis gave warning to the Roman public that a retaliation was imminent.