The long-drawn-out World War impacted on the operation of Jämsänkoski mills mostly as shortage of foreign materials and spare parts. The papermaking machines stood idle for part of the year 1917. In 1918 paper production was less than half the 1915 level. Only a third of the quantity of a normal year's output of pulp was produced. Jämsänkoski had joined the Association of Finnish Paper Producers, and paper sales were conducted through this channel. Jämsänkoski did not join the Association of Finnish Pulp Producers, as the paper mills used all the pulp produced for themselves.

Jämsänkoski mills management in the mid-1910s consisted of the Superintendent, Jakob Solin, technical director of the pulp mill, Onni Pyykkö, technical director of the paper mill, Bernhard Ståhle, and the administrative manager, Captain Alfred Hartman. The cashier was Henrik Solin. Forest industry companies set up dummy companies in order to get round the 1915 land acquisition legislation, which limited their right to purchase farms. Maanviljelysosakeyhtiö Päijänne was established in Jämsänkoski, with all its shares owned by Oy Jämsänkoski. The parent company lent the funds for purchasing the farms, and Päijänne then leased the estates it owned back to Oy Jämsänkoski.

The Myllykoski and Simpele mills led by Rudolf Walden did not have their own pulp production. As the price of cellulose was rising, either building of its own plant or acquisition of a suitable operational plant became topical. In December 1919 Ab Simpele and Myllykoski Träsliperi Ab acquired the majority shareholding in Jämsänkoski. Walden charged Engineer Anton Kuhn and Myllykoski Superintendent Benjamin Snellman with the task of assessing the condition of the Jämsänkoski plants.

Benjamin Snellman was appointed new Superintendent in place of Jakob Solin. At the April 1920 Annual General Meeting, the capital stock of Oy Jämsänkoski was raised to eight million markka. The share dividend for the previous year was 10 percent. The early part of the year was spent estimating the assets of the merging companies, Jämsänkoski, Simpele and Myllykoski. In June each company held an Extraordinary General Meeting. All approved the amalgamation of the companies under the company name Yhtyneet Paperitehtaat Osakeyhtiö (United Paper Mills Ltd), and its Certificate of Incorporation was approved by the Council of State on 28 July 1920.