I want to know what were the names of the Popes at the Start of World War1 and World War 2?

If u go to this site it should answer the question.
www.newadvent.org

Actually, the site doesn't, not in their list of Popes, anyway. It seems that both in 1914 and 1939, there were 2 Popes, one elected after the death of the previous one. But no dates of death are given; only the years of death are listed.

St. Pius X died in 1914, and Benedict XV became Pope.

Pius XI died in 1939, and Pius XII became Pope.

Someone else will have to determine the dates of deaths and elections to see who was Pope at the time each war actually started.

DG
The tricky one date wise is Benedict XV who became Pope on September 1 1914 http://en.wikipedia.org/wiki/Pope_Benedict_XV

His predecessor( Pope Pius X)was still alive at the outbreak of war July 28th
details here

http://en.wikipedia.org/wiki/1914

Pope Pius XII succeeded on March 2, 1939 till 1958
http://en.wikipedia.org/wiki/Pope_Pius_XII
NOTE Found a note in Wikipedia for 1939 that says :
March 15 - German troops occupy the remaining part of Bohemia and Moravia; Czechoslovakia ceases to exist
WW2 Officially started September 3rd
Another close call?!

Good one, Bedstor! Made me work to no avail; spent a few hours trying to research this one, because of the lack of dates as Dorian pointed out; couldn't find the dates of death or election!