MapHisto uses a number of sites on the internet. The most important are:

Contains a huge amount of information. Especially the German and the English versions have been used but also the Danish, Swedish, Norwegian and the Dutch have been used.

Natural Earth
Natural Earth is a public domain map dataset available at 1:10m, 1:50m, and 1:110 million scales

World Statesmen
World Statesmen. Lots of facts.

Deutsches Reich 1871-1918
Great site on the German Empire.

Statistiches Bundesamt Deutschland
German population numbers.

Posselts map of Germany from 1895.

HGIS Germany
Historical-geographical site about Germany 1820-1914.

History of Swedish cities.

Population Statistics
World population figures.

Centraal Bureau voor de Statistiek
Dutch population figures.

Statistiska Centralbyrån
Swedish population figures.

Statistisk Sentralbyrå
Norwegian population figures.

Danmarks Statistik
Danish population figures.

Statistics Estonia
Estonian population figures.

A vision of Britain
British population figures.

Angus Maddison
World historical population figures.

Statistični urad Republike Slovenije
Slovenias Bureau of Statistics.

Sources to the borders

Northern Scandinavia: The basis for the Swedish Norwegian border is the modern border. The borders for Härjedalen and Trondheim are found in Historical Atlas by William Shepherd (1911).

Sweden and Finlands border to the Sami people: Tacitus.

Crimea: The borders for the Crimean Khanate and the possessions of Genova are found in: Wikipedia - History of Crimea

France during World War II: Wikipedia - Vichy France and Wikipedia - Italian occupation of France

Montenegro 1852-78: Wikimedia - H. Kiepert's neue Karte von Montenegro, 1862

Moldavia 1484-1812:

Greece 1832-1847: Wikimedia - Greek Territorial Gains 1832-47 and Stieler's Handatlas 1891 map 50.