A gang war is certainly less of a war and it’s participants less violent precisely because it is not a national conflict. Lets take Mexico as our example since it’s drug conflicts are among the largest scale gang war seen to date. Estimates of the Los Zetas cartel place its 2013 membership at around 3000, with the Sinaloa coming in around the same. While some lament the access of drug cartels to military hardware, the idea that any criminal cartel will ever possess the firepower of any modern military. Even with their assult rifles and RPG’s the Zetas remain unsuprisingly bereft of Artillery, Tanks, and Military Aircraft that have caused the lions share of casualties in 20th century war. When organizations do get these weapons, you get situations like the ones in Syria, with entire city blocks leveled and tens of thousands dead. A gang in a gang war will never have the training, motivation, and most importantly the supply lines to do violence on the same skill as even the most backward modern military.

PS: There was a lull in central European conflict in the 10 years prior to WW1, but certainly nothing global. There is also the fact that it was hard to have more than one serious war a generation in the late 19th and early 20th centuries. If a significant percentage of your fighting age population is dead or disabled, peace tends to occur out of sheer practicality. Outside of central europe though, conflict continued as normal.

1899 Philipine Insurrection

1900 Boxer rebellion in China

1904–1907 Russo-Japanese War begins

1911 Tripoli War between Italy and Turkey begins

1911 (until 1912) Chinese Revolution begins at Wuhan

1912 First Balkan War begins; Montenegro declares war on Turkey, soon joined by Bulgaria, Greece, and Serbia