Which country launched a surprise offensive attack that became known as the Battle of the Bulge?

Germany.
The Battle of the Bulge (aka the Ardennes Counteroffensive) was a fast offensive attack against the Allied forces in the Ardennes Forest. They took advantage of detrimental weather, moving undetected through the forest since the Allies’ air forces were grounded. After a month of fighting however, the Allies were victorious. When their air forces started flying again, bombardments on key German sites only pushed the Germans further and further towards defeat.