Britain was also worried about Germany's growing military power. Germany was developing its large army into one of the most powerful in the world. The British Army was quite small. The British Royal Navy was the largest and best in the world, and in the 19th century that was enough to keep other naval powers from attacking. Germany was a land power, and Britain was a sea power. But now the Germans were building a large navy. This was seen as a threat to Britain. However, the decision to declare war was taken under its alliance with Belgium in the Treaty of London (1839). The Government might have decided differently. No-one foresaw how long the war would last, and what the terrible costs would be.