The expedition crossed the border on the night of April 21, 1919. The goals were to capture Lodeynoye Pole, Petrozavodsk and the Murmansk railroad. The troops were divided into three groups and were made up of 1000 volunteers. The southern group advanced to Lodeynoye Pole in just three days, but was pressed back behind River Tuulos by Bolshevik troops. The northern group captured Prääsä. At this time it became obvious that there weren't enough troops to complete the goals of the expedition. A new round of recruiting 2000 new volunteers was started and Mannerheim made Aarne Sihvo the new commander of the expedition. Major Paavo Talvela's regiment started an attack aimed at Petrozavodsk on June 20, but was beaten by Red Army and Finnish Red Guard forces just outside the town. The British troops that operated along the Murmansk railroad were quite close by, but did not participate. The Finns had hoped that the Karelian population would have joined the troops as volunteers but only a few did and their morale was never very high. The initiative now passed to the Bolsheviks. On June 26 over 600 Finns of the Red Officer School in Saint Petersburg made a landing at Vitele across Lake Ladoga behind the Finnish lines. The southern group was forced to retreat to Finland after suffering heavy losses. Talvela's group was also forced to retreat back to Finland.
Question: Who did the south retreat
Answer: after suffering heavy losses