The 20th century saw the most destructive wars in human history; probably with the greatest loss of life ever known. In reviewing the period for a history timeline I do not know where to begin. In my mind the destructiveness of the period overwhelms the great progress in science and health for those fortunate enough to live in countries where they are able to enjoy these benefits.
In the first world war more than 65 million combatants were mobilised with more than 8.5 million killed. Total casualties including deaths, wounded, missing or prisoners were 37.5 million. I have not found the information on civilian deaths. The total cost of this war is estimated at 186.333 BILLION U.S.dollars! (1914 - 1918 prices) Australia spent 1.4 Billion. Another interesting fact about that period is the number of deaths from the Influenza pandemic which hit in 1918 at the end of the war. India alone had 16 million deaths and world wide there were more than 70 million deaths. Many soldiers also died from the disease which it is postulated was brought to the trenches by American soldiers.
Statistics which I have found for WW2 indicate that a total of 55 million person from both sides, including civilians, were killed. Interesting to note that in Germany 3.8 million civilians were killed against 60,000 in the UK. Statistics for those killed often quote only Allied losses (about 43 million) and do not include the Axis (enemies) deaths. As well as the deaths there would have been many wounded and injured; both civilian & military. I have not yet found informtion on the cost of WWII. In the aftermath of WWII the millions of displaced persons, deaths from starvation and the partition of Europe brought additional misery to the population of those countries.
If you consider the enormous amounts spent in carrying out war and then reconstruction after they are over, it makes a person wonder what could have been achieved using those resouces to improve living standards around the world.
It is also the period in which the environmental impact of a burgeoning world population began to make itself felt with the increasing destruction of the natural world on which we depend. In addition there was the reduced ability of the planet to sustain us because of the effects of pollution and other pressures on the world environment.