You could read a few history books to learn how the assassination of an Austro-Hungarian archduke led to the start of the Great War, how that war killed so many combatants, how the eventual peace treaty sowed the seeds for a Second World War, how the Russian Revolution resulted in worse oppression than that which the revolution was supposed to correct or how Prohibition in the United States fueled organized crime. Or you could read Ken Follett's massive, yet always interesting, novel Fall of Giants, the first volume of his Century Trilogy.
Follett explains these historical events through the lives of fictional characters in Great Britain, Getmany, Russia and the United States. Through their lives, we experience something of the lives of millions of people actually living during the years from 1911 to 1924, the span of time covered by Follett's novel. Real historical figures, such as Woodrow Wilson and Winston Churchill, also play roles in this drama.
Coincidences happen in real life, but not to the extent they happen in Fall of Giants, where main characters seem to be running into each other everywhere, even on the battlefield. As unrealistic as this may be, it does make the story easier to follow and eliminates the need for scores of additional characters. The scores we already have are plenty.
A more serious problem with the novel is that Follett attempts to turn it into a 920-page political tract. All of the characters portrayed positively share the same political ideas, as do all those portrayed negatively. In real life, there are intelligent and noble individuals at both ends of the political spectrum.