German theories of history clearly played a significant role in the work of Gardiner. However Gardiner was interested not just in German theories of history, but also in German history, both as a part of the story that he told of seventeenth-century England, and in its own right. In the 1870s, perhaps as a result of the Franco-Prussian War, the subject of the internecine European struggles of the seventeenth century loomed large for Gardiner's conception of the political and religious crises in England in that century. As a result, the Thirty Years War plays an important part in the History of England and is dealt with in great detail, supplied at least in part by the British historian's use of the work of German historians. Furthermore, although Gardiner's interest in that war waned in later years, in 1874 he published a book in Longmans' Epochs of Modern History series of introductory texts, entitled The Thirty Years' War, 1618–1648. A brief summary of the role of German historiography in Gardiner's main work, and a more detailed analysis of his own study of German history is therefore in order as a complement to his theoretical preoccupations and to provide a more nuanced account of his work.

Gardiner's History of England contains footnote references to at least thirty-six separate German-language works of history. Although these include books concerning European countries other than Germany, such as von Ranke’s work on French history, Julius Opel’s Der Niedersächsisch-Dänische Krieg (1872–94) and Adolf Buff’s Die Politik Karls des Ersten in den ersten Wochen nach seiner Flucht von London und Lord Clarendons Darstellung dieser Zeit (1868), the majority are concerned with German history.