The Admiralty was much less concerned with the USN's build up than they were with (hypothetical) French or (actual) German naval expansion. There were several reasons for this. Firstly, a strong USN was much less of a direct threat to Britain than an equally strong European navy. A French or German navy that could challenge the Royal Navy threatened an invasion of Britain. A strong American navy could threaten Canada, but had no bases on the eastern side of the Atlantic from which to invade Britain. The European powers also had strong armies, exacerbating this threat. Secondly, the USN, unlike the European powers, was a 'two ocean navy'. It had to protect coastlines in both the Atlantic and the Pacific. This meant that it was harder for the USN to create a force that could truly challenge the RN. Should a war occur, the RN could defeat the USN in detail, destroying its Atlantic fleet, before facing the Pacific fleet. As a result, the USN would need to have a fleet that could do real damage to the RN in both oceans, something that was out of reach for much of the 1900s-1910s. Finally, and this was especially true after WW1, there was a general understanding within the Admiralty that Britain could not afford to outspend the USN. The war had greatly damaged the British economy, with it becoming burdened with unsustainable levels of debt. Spending money on a new naval arms race was not in British interests, and so there were moves towards limiting the US's buildup in different ways. The Washington Naval Treaty was the ultimate outcome of this, with Britain accepting equality in tonnage with the USN.