The directive stated that "operations should now be focused on the morale of the enemy civilian population, and in particular, the industrial workers". Lest there be any confusion, Sir Charles Portal wrote to Air Chief Marshal Norman Bottomley on 15 February "...I suppose it is clear that the aiming points will be the built-up areas, and not, for instance, the dockyards or aircraft factories". Factories were no longer targets. The first true practical demonstrations were on the night of 28 to 29 March 1942, when 234 aircraft bombed the ancient Hanseatic port of Lübeck. This target was chosen not because it was a significant military target, but because it was expected to be particularly susceptible – in Harris's words it was "built more like a fire lighter than a city". The ancient timber structures burned well, and the raid destroyed most of the city's centre. A few days later, Rostock suffered the same fate. At this stage of the air war, the most effective and disruptive examples of area bombing were the "thousand-bomber raids". Bomber Command was able by organization and drafting in as many aircraft as possible to assemble very large forces which could then attack a single area, overwhelming the defences. The aircraft would be staggered so that they would arrive over the target in succession: the new technique of the "bomber stream". On 30 May 1942, between 0047 and 0225 hours, in Operation Millennium 1,046 bombers dropped over 2,000 tons of high explosive and incendiaries on the medieval town of Cologne, and the resulting fires burned it from end to end. The damage inflicted was extensive. The fires could be seen 600 miles away at an altitude of 20,000 feet. Some 3,300 houses were destroyed, and 10,000 were damaged. 12,000 separate fires raged destroying 36 factories, damaging 270 more, and leaving 45,000 people with nowhere to live or to work. Only 384 civilians and 85 soldiers were killed, but thousands evacuated the city. Bomber Command lost 40 bombers. Two further thousand-bomber raids were conducted over Essen and Bremen, but neither so utterly shook both sides as the scale of the destruction at Cologne and Hamburg. The effects of the massive raids using a combination of blockbuster bombs (to blow off roofs) and incendiaries (to start fires in the exposed buildings) created firestorms in some cities. The most extreme examples of which were caused by Operation Gomorrah, the combined USAAF/RAF attack on Hamburg, (45,000 dead), attack on Kassel (10,000 dead), the attack on Darmstadt (12,500 dead), the attack on Pforzheim (21,200 dead), the attack on Swinemuende (23,000 dead) and the attack on Dresden (25,000 dead).