General Gustaf Mauritz Armfelt, now headquartered at Lier, hoped to take the strategically important fortress through a pincer movement with the help of Gahn's "Flying Corps".

On the evening of 24 April Gahn crossed the Swedish-Norwegian border with the 2nd battalion of the Dalarna Regiment, and moved westwards, along the southern side of Flisa River.

At dawn, the battalion was near the Flisa River, where it faced the first Norwegian outposts, who immediately fired warning shots and sent out a report about the Swedish advance.

[4] The defending troops, commanded by Colonel Bernhard Ditlef von Staffeldt, had begun establishing a defense in the area in early April 1808.

He found it highly unusual that the Swedes marched into Trangen, a narrow pass between the cliffs of Kjelsås and Buttenås, without sending troops forward along the main road on the north side of the river, and therefore feared that it was a trap.

He nevertheless sent 270 men under the command of Captain Elias Nægler to the defensive position at Trangen, which had been prepared with roadblocks of trees, and where it was likely that the Swedish troops would pass.

From Trangen, Captain Nægler could hear the sound of the heavy fighting east of them, and he choose to take his grenadiers and march quickly in the direction the Swedes had retreated.

[9] Colonel Gahn's thrust against Kongsvinger Fortress came to be the last offensive maneuver the Swedes carried out south in Hedmark until they withdrew back to Sweden in late May.

[5] In retrospect, and especially during the Norwegian romantic nationalism in the late 19th century, emphasis was placed on Nicolay Peter Drejer's courageous efforts and he was given much of the credit for the victory at Trangen.

The event was attended by King Harald V, who also visited Sønsterud Farm and the room where Captain Drejer had died, which had been restored for the anniversary.