Britain declared war on Germany after Hitler invaded Poland and refused to withdraw his troops. Thousands of Jewish and other children were evacuated from cities to rural areas for safety, being told it was a holiday, as they would likely end up in concentration camps if they remained. During air raids known as the Blitz, people used bomb shelters like the Anderson shelter, a small corrugated iron structure dug into a hole in the ground, to protect themselves from Hitler's heavy bombing of Britain. They also had to observe blackouts by shutting all curtains and switching off lights at night and use gas masks in case of poison gas attacks.