This problem was compounded when the main Hanoverian army under Cumberland, which include Hesse-Kassel and Brunswick troops, was defeated at the Battle of Hastenbeck and forced to surrender entirely at the Convention of Klosterzeven following a French Invasion of Hanover. The convention removed Hanover from the war, leaving the western approach to Prussian territory extremely vulnerable. Frederick sent urgent requests to Britain for more substantial assistance, as he was now without any outside military support for his forces in Germany.