Overview: On 24 February, Russia's president, Vladimir Putin, ordered a military invasion of neighbouring Ukraine, triggering the largest refugee crisis in Europe since the Second World War. Almost six million Ukrainians have fled the country, half of them to next-door Poland. During a week in April, Our World follows Warsaw's mayor, Rafal Trzaskowski, as well as refugees and volunteers living through the crisis. How have the people of Warsaw dealt with such a sudden increase in the city's population. And what is the long-term plan for all the new arrivals?