As a result, the Whites and Germans captured about 30,000 Reds and their family members who were placed in a concentration camp on the outskirts of Lahti.

The town was important for the Reds due to its location by the vital Riihimäki–Saint Petersburg railway, connecting Lahti to the major war theatres in Tavastia and Karelia.

In the late April, there were about 40,000 Reds inside the triangle formed by the towns of Hämeenlinna, Riihimäki and Lahti.

After the victorious Battle of Helsinki, fought 12–13 April, the division marched north to Riihimäki and Hämeenlinna, which forced the Red refugees to head to Lahti.

The original plan for Detachment Brandenstein was to attack the Red stronghold of Kotka and then cut the Saint Petersburg railway in Kouvola.

[2] As the Germans reached Lahti along the Loviisa–Vesijärvi railway, the Reds started building trenches, artillery batteries and other defensive posts to the slopes of the Salpausselkä ridge.

At the same time the Finnish White Army division, commanded by the Estonian major Hans Kalm, entered the town from north as the Reds on the Radiomäki hill had surrendered.

[4] The German commander colonel Otto von Brandenstein and Hans Kalm greeted each other in a modest ceremony held in the Lahti main street.

[3] On the next day, a small clash occurred as a group of 1,000 Red Guard fighters came by armoured train to Okeroinen, a village in Hollola, 5 kilometres south of Lahti.

They were desperately trying to break the German lines and march through the town to continue their journey eastward, but managed only to take the Hennala Garrison.

The fighting lasted for two days but despite their overwhelming strength, the Reds could not beat the experienced and well-armed German troops.

There were only a couple of organized units, like the Turku Women's Guard and the squad composed of the youth section of the sports club Jyry Helsinki.

At 8:00 a.m., the Reds started surrendering and the Battle of Lahti was over, although there was still minor resistance in the surroundings of the town and five Germans were killed on 2 May.

Among the captured was also the group of 4,000–5,000 Red refugees who had only a couple of days earlier fought their way through the German lines in the bloody Battle of Syrjäntaka.

This was verified in the war diaries of the German officer Hans Tröbst, released as the sixth part of the 2015 book Der Krieg im Westen.