In pictures: Russia invades Ukraine
After months of military buildup and brinkmanship, Russia launched an unprecedented assault on Ukraine in late February. Explosions rang through a number of Ukrainian cities, including the capital Kyiv, in a broad invasion that began before dawn on February 24. Fighting continues to take place throughout the country, and so far the Ukrainian resistance has been able to hold Kyiv and many other key cities. Meanwhile, more than 4.5 million refugees have fled Ukraine to neighboring countries, according to the United Nations' refugee agency. The United States, the European Union and other Western allies have been rolling out sanctions in…