The 2nd World War came on the heels of the Great Depression. Germany rose to a world power again under the Nazi regime of Adolph Hitler. Mass execution of Jews, Gypsies and anyone else that did not fit into Hitler's "master race" were placed in concentration camps and murdered by the millions. Germany invaded Poland and quickly conquered most of Europe. Great Britain and the Soviet Union, along with resistance fighters in France, The Netherlands and other nations were left to battle Germany and its allies. Japan was quickly taking over the Pacific.
On 7 January 1941, Japan attacked the US naval base in Pearl Harbor, Hawaii, bringing America into the war.
The allied D-Day invasion on 6 June 1944 of Normandy began the downfall of Hitler's Germany. 1945 would see the war come to an end. Germany surrendered in May. Rather than risk an invasion of Japan that would lead to upwards of a million allied casualties, the US chose to drop atomic bombs on the Japanese cities of Hiroshima and Nagasaki in August, leading to Japan's surrender.
The internet has a plethora of terrific sites on WWII, check them out. For a timeline of events see
Family connections: Uncle Charles McHugh served stateside as an instructor, I believe, with the engineers. John W. Simmons Jr. fought with the Philippine Resistance and was killed by the Japanese in 1943.