The fare war we've seen between Delta and United the past few days has now expanded accross the European continent. Fares range from the mid $400s to the mid $500s round-trip. Some flights may be operated by partner airlines.
This sale has been limited to major European destinations for a few days, but great deals from Delta and United hubs in Atlanta, New York, Washington D.C., Detroit, Chicago, Houston, Minneapolis, Denver, Salt Lake City, San Francisco, Seattle, Cleveland, and Cincinnati can now be found to an extensive list of cities in Europe.
Fly to Athens, Dublin, London, Madrid, Budapest, Istanbul, Warsaw, Helsinki, Prague, Tallin, Estonia, Oslo, Sofia, Bulgaria, Belfast, Bucharest, Stockholm, Kiev, Zagreb, Croatia, etc. That is not a complete list, just naming a few. Basically, nearly everywhere in Europe is on sale from the Delta/United hub cities listed above and from some surrounding airports as well.
This is unadvertised and may not last very long. Book quickly if something catches your eye. Most third party booking sites have a 24-hour free cancellation policy so you'll have a day to figure out the details.
Dates may vary by route, but generally for travel through mid-May and from mid-August onward. A 28-day advance purchase and a 7-night minimum stay are required.
Don't live near one of those cities? You're still in luck. The previous sales from nearly every airport in the USA to major western European airports such as Amsterdam, Paris, Rome, Milan, Venice, Lyon, Brussels, Munich, Frankfurt, Berlin, Geneva, Luxembourg, Zurich, and Vienna, etc. are still valid at time of posting.
Check flights from your city, within the travel dates listed above, to see what's available from your neck of the woods. Even if your favorite airport (small or large) isn't listed, there may be a big price drop. Other cities in these countries are likely lower than normal as well.