Short answers, pulled from the story.
Four empires collapsed because of World War I: the German Empire, the Austro-Hungarian Empire, the Russian Empire, and the Ottoman Empire. Their dissolution led to the creation of numerous new states across Europe, the Middle East, and the Caucasus.
The Treaty of Versailles, signed on the 28th of June 1919, required Germany to accept responsibility for the war, pay reparations of 132 billion gold marks, reduce its army to 100,000 men, and give up territories including Danzig and the land forming the Polish corridor. Germany was also forbidden from possessing tanks, warships, warplanes, and submarines.
About 50 million people are estimated to have died from the 1918 influenza outbreak worldwide. A 2005 study found that the virus strain developed in birds, similar to 21st-century bird flu. A British research team led by virologist John Oxford identified a major troop staging camp in Etaples, France as almost certainly the center of the pandemic.
Italy's dissatisfaction with the peace settlement, which Italians called the mutilated victory, fueled nationalist resentment that Benito Mussolini exploited. Gabriele D'Annunzio's 1919 seizure of Fiume introduced the title Il Duce and the black-shirt paramilitary uniform, both later adopted by Mussolini's Fascist movement. Mussolini founded the National Fascist Party in 1921 after his Fasci di Combattimento suffered a heavy defeat in the November 1919 elections.
The 1921 United Kingdom census recorded 1.72 million more women than men in England and Wales, a gap newspapers called the Surplus Two Million. There were 1,209 single women aged 25 to 29 for every 1,000 men. By 1931, half of these women were still single and 35% did not marry while still able to have children.
At the 1919 Paris Peace Conference, China's request for the return of Jiaozhou Bay, under German control since 1898, was rejected, with its rights transferred to Japan instead. China refused to sign the Treaty of Versailles and signed a separate peace with Germany in 1921. The Western Allies' decision to favor Japan at China's expense directly triggered the May Fourth Movement, which is often cited as the birth of Chinese nationalism.