Petkeljärvi´s varied terrain is interspersed with large and small lakes, and is closely associated with the Winter War in Ilomantsi. The national park was founded in 1956. 

Fighting extended to the vicinity of Lake Petkeljärvi after the Front had stabilised at Oinassalmi. Sections of wartime trenches remain visible on ridgelines within the park, while restored fighting positions and the site of a large dugout can be examined near the Outdoor Centre. 

The fortifications were constructed before the Winter War between Finland and the Soviet Union from 1939 to 1940. A general mobilisation under the guise of "additional refresher training” was declared in October 1939. In Ilomantsi, reservists, Civil Guard members and active-duty border troops were called to arms beginning on 6 October. 

Underaged youngsters and older men who were not eligible to take up arms but could be called to compulsory labour service were also ordered to participate in the fortification effort. Under the direction of active-duty frontier guardsmen, they immediately began to prepare defensive positions along the road leading to the current park area and in the vicinity of the site of the present Outdoor Centre. 

The Winter War began on 30 November  1939, and enemy forces reached Oinassalmi by mid-December. The Finnish eventually halted the Russian advance on the eastern shore of strait after blowing up the road bridge, and the Front stabilised for the remainder of the Winter War. No major engagements were fought within the boundaries of the national park, except for one fierce clash when the Russian forces attempted to break through to the rear of Oinassalmi’s defenders. Both sides mounted regular patrols within and around the area to obtain information of the situation along the contact line. Finnish troops that had entrenched on the ridgelines provided fire support to the defenders at Oinassalmi and suppressed enemy forces that held the terrain on the other side of the narrows. 

Sporadic firing continued until the end of hostilities on 13 March 1940.