Modern area built in Rome pre WWII

Perhaps you're thinking of EUR.

Probably EUR. It's a residential and business area - not at the top of my list to see (actually it's not even on the list), but if you want to see Mussolini-era architecture, there it is.

Yes, this would be the EUR. If you are interested in pre-WWII era architecture and a taste of what Mussolini envisioned for the future city, it's a good visit. It's an easy trip: there are two subway stops for EUR (both equidistant from Columbus Blvd). Walk up Christopher Columbus Boulevard just past the main Piazza Marconi and the main complex is to the left. There are some museums at Piazza Marconi. The "town" area is closer to Metro stop EUR Palasport if you want to explore a neighborhood or find a restaurant. That same Metro line has the stop Piramide which is just outside the ancient walls. A couple of Roman sites to see there, including a Pyramid tomb built into the walls.

For that matter, Ostiense Station is a good example of pre-war fascist architecture. It was built by Mussolini 1938-40 to commemorate an upcoming visit by Hitler. A block north is Piazza San Paolo, where on 10 September 1943, Italian partisans battled German troops invading Rome from the south. The Pyramid of Cestius, impressive in itself, is right there.

In addition to the EUR (Google Maps link here: https://goo.gl/maps/u6CkPVjtdyC2), you find fascist era architecture sprinkled lots of places in Rome. In addition to the Ostiense station (link here: https://goo.gl/maps/PcRLcc6VRhD2 ), I'm thinking of the Foro Italico, near where the Italian Open tennis tournament is held. There are mosaic sidewalk designs of crowds giving the fascist salute and chanting "Duce Duce" next to an obelisk that still has Mussolini's name on it. Here's a Google Maps link:
https://goo.gl/maps/UFQJ7gkC2252

Here's the obelisk: https://goo.gl/maps/4KET4mkMP4n

There's several modern areas in Roma. As a matter of fact, the center is very small compared to the built surface of the city and there are, specifically, a number of districts built before WWII (or right before WWI, for that matter). Other posters have already mentioned the EUR. In case it isn't the one that was mentioned to you, consider googling the Quartiere Coppedé, easily reachable by subway (the closest stop is Policlinico) + a short tram ride going North on either the 3 line or the 19 line: this is an Art Nouveau district built starting in 1915.
Other singular districts are:
- Garbatella (get off at the subway stop with the same name + walk down Circonvallazione Ostiense & around the newest buildings you'll see at the subway exit), which was visited by none other than Gandhi who was, well, a fan.
- Città Giardino (by piazza Sempione, off via Nomentana)
- Villaggio Olimpico: opposite the already mentioned Foro Italico, this district was in part designed during the war and only built for the 1960 Olympics.
- Pigneto: south of Termini, reachable by tram (lines 5 and 14). Look up the area within this district called "Villini", built between 1921 and 1924. It means, literally, "little villas", and they're mostly found around piazza Copernico.