The Kellogg-Braind pact was an agreement to not start war with other nations that they may have an argument with. Instead they must settle with peace, if any conflict comes up they must not resolve with violence.
By 1928 the pact had 15 nations signed and agreeing to no violence. It started with the United States, Germany, and France.
Japan got into invaded Manchuria, breaking the Kellogg-Braind Pact. Followed by Germany invading Poland, also breaking the pact.
Germany invaded Poland because Hitler was trying to take over the region. He wanted it to be "living space" for Germans. This started WW2 because Hitler wanted to spread his belief, a communist government.