Carlo explains that he's a secretly guilty man. He says that he was chosen for the mission because he's reasonably intelligent and a good soldier. He followed orders to take one reliable man and report to Colonel Rivolta. Rivolta's headquarters are at an abandoned villa where he can hold parties. He greets Francisco and Carlo with a Roman salute, which makes Francisco giggle. Rivolta dramatically calls them to his map, points to a spot in Greek territory, and tells them that brigands in disguise have control of the tower. He wants them to take it.
Note that Carlo makes sure to describe Rivolta's headquarters as being suitable for parties. This suggests that the higher levels of the Italian army are all corrupt and more interested in looking fancy and powerful than they are in actually doing anything meaningful. Rivolta's dramatic attitude reinforces this, as it suggests he'd like to think he's powerful and special.
Francisco is shocked that they're being asked to go into Greek territory and Mario makes matters worse by trying to climb out of Francisco's pocket. Rivolta, thoroughly annoyed, tells Francisco not to question policy and sends them away with the promise of medals. In private, Francisco insists that this mission is fishy and dirty. This is confirmed when the quartermaster gives them British uniforms and Greek weapons, including a machine gun with no instructions. At 10pm, they creep out of their camp. Carlo explains that they were extremely nervous, and implores the reader to understand that the atrocities soldiers commit often feel, to the anxious soldiers, like catharsis.
Remember that Italy hasn't yet declared war on Greece; Francisco is right that this is fishy. The combination of British uniforms and Greek weapons reinforces this and suggests that these two are being asked to fabricate one of the border incidents, which as a whole allow Mussolini to make it seem as though Greece is an aggressor. Carlo's explanation of why the atrocities can feel good for soldiers reminds the reader that soldiers are human and susceptible to human emotions like anyone else.
Carlo and Francisco lie in the scrub near the tower. Near midnight, a freezing wind picks up and Francisco suggests they attack early so they don't die of exposure. Carlo agrees. They discover ten men with machine guns in the tower. The brigands have a drum of kerosene underneath, so Carlo overturns it and sets it on fire. They fire the machine gun upwards and feel no triumph when they're done. Francisco discovers the body of an Italian captain, carrying orders to be on the lookout for an attack by the Greeks at 2am. Carlo realizes the Greeks wear British uniforms--they were supposed to die as Greek soldiers in British uniform. Francisco bitterly says that someone wants to provoke a war with Greece.
Notice that Carlo and Francisco only "succeed" because they choose to go against orders to protect themselves. This shows them that they're disposable and not actually a valued part of the Italian army, as they may have thought once. Rather, they're pawns for people way more powerful to use for their own petty means--which, it should be noted, will send many more Italians to their deaths.