Frederick's invasion of Saxony in 1756 was used as a pretext for war, being denounced by both Sweden and France as a violation of the Treaty of Westphalia of 1648, of which they were both guarantors. On March 21, 1757 the governments of France and Austria agreed to a convention in which Sweden and France explained that they had to maintain Germany's freedom in line with the treaty. France promised financial backing for such a war and in June the same year the decision was made to send 20,000 Swedish troops to Germany to emphasise their commitment. On September 13 that force invaded Prussian Pomerania.