United Paper Mills Ltd, formed from Simpele, Myllykoski and Jämsänkoski mills, was established in 1920. Rudolf Walden owned one third of the shares in the new company. Jämsänkoski was an important element of the corporation, due to the pulp it produced. Consequently, one of the first reforms undertaken was increasing pulp production. Benjamin Snellman was selected to take charge of the reforms. Technical design was executed by a German engineer, Anton Kuhn.

The most visible changes were the many construction projects; the new acid tower, extension of the repair workshop, and building of accommodation for workers in the summer 1920. A lot of outside workers came to Jämsänkoski and the projects exceeded the planned limits. The approved funds were insufficient, and it soon also turned out that not all building work had been done well enough. Engineer Kuhn's reforms collapsed through overambitious plans, lack of supervision and technical incompetence. The building boom was later referred to in Jämsänkoski as the Million Summer.

Everything changed when Rudolf Walden returned from the Tartu peace negotiations and familiarised himself with the situation at Jämsänkoski. The company had suffered a large loss in 1920, blamed on incompetence of local management. Those responsible were dismissed. All in all, more than thirty men left Jämsänkoski in spring 1921.

After the lessons learned in the Million Summer, Rudolf Walden ruled his company with an iron hand. A new technical director, Roope Hormi, was appointed from Simpele mill, and he continued the reforms at Jämsänkoski in stages. The most important were increasing pulp production and reforming energy supplies. The results also showed; output of the pulp mill had tripled by the end of the 1930s compared to the year 1920. The paper mills concentrated on fine grade papers which commanded a better price. Paper production grew by 40 percent. The groundwood plants were discontinued and all water power was harnessed for electricity generation.

Rudolf Walden's time at the helm of United Paper Mills is known as the General's period. It was Walden's view that minimising production costs was the way to improving profitability of the company. Building works and raw materials purchases had to be curtailed and demands for wage rises rejected. Walden made regular visits to individual factory units, and looked into their affairs in great detail. These visits were usually a source of fear in the locality, as the General's beady eye missed nothing. In 1925, Veli Byström was appointed Superintendent at Jämsänkoski from his previous similar position at Simpele. Byström worked under Walden's strict supervision, and during the depression years of the early 1930s even the Superintendent had to give up his company car.

Within the company, the Jämsänkoski reforms were overshadowed by the big building projects in the 1930s at Myllykoski. Oy Walkiakoski was officially amalgamated with the company in 1934, and by the end of the decade United had grown to become one of the largest manufacturers in the country. The General's 60th birthday was celebrated with a flourish in all United mill towns in 1938.

Even before the outbreak of the Winter War, United had made preparations by reducing production. In the final stages of the Winter War, all production from the paper mills was sold in the domestic markets. Only 26 percent of the mills' capacity was in use. During the Winter War, General Walden acted as representative of Mannerheim and his headquarters to the Government. Walden was especially active in obtaining foreign help for Finland at war. In March 1940, Rudolf Walden was appointed Defence Minister. His eldest son, Juuso Walden, was appointed new Managing Director of United.