 During the Second World War, the protection of certain Thames-side facilities, particularly docks and water treatment plants, was crucial to the munitions and water supply of the country.The river's defences included the Maunsell forts in the estuary, and the use of barrage balloons to counter German bombers using the reflectivity and shapes of the river to navigate during the Blitz.In the post-war era, although the Port of London remains one of the UK's three main ports, most trade has moved downstream from central London.