Did fighter planes or tanks get better? What about gas? The geneva conventions weren't until 1949, if I recall correctly, so why was it less prevalent?
[Warfare tactics changed greatly between World War One and World War Two; when and why did this occur?] (http://www.reddit.com/r/AskHistorians/comments/2r7ac7/warfare_tactics_changed_greatly_between_ww1_and/)
[How different was maneuver warfare in 1918 from 1914?] (https://www.reddit.com/r/AskHistorians/comments/33eelp/how_different_was_manoeuvre_warfare_on_the_ww_i/)
[Why was WWII more mobile than WWI?] (http://www.reddit.com/r/AskHistorians/comments/3bd8ns/why_was_world_war_2_more_mobile_than_world_war_1/)
These answers I've given should point you in the right direction.
In short:
wireless radios allowed commanders to more effectively control their forces, allowing for opportunities to be swiftly exploited
the development of armoured fighting vehicles and motorized/mechanized transport allowed for mobile forces more effective than cavalry to carry out decisive actions and exploitation
more effective aircraft could coordinate advances, provide air support to ground troops, and cut off tactical and operational reserves from the battlefield/zone, thus preventing breakthroughs from being parried
Trenches became obsolete in early 1918 at the latest. Through years of experimentation both sides fighting in WW1 had found new tactics that worked against heavily fortified areas. This included:
Short and highly precise artillery barrages and following them up immediately with infantry to catch the enemy off guard, instead of the "old" style of doing very long bombardments that let the enemy prepare ac ounter-attack.
Small group infantry tactics. I.e. much more flexibility and freedom to act being given to NCOs and even enlisted men to do whatever they thought needed doing without having to wait for new orders to reach them. They also introduced a "leapfrogging" style of advancing under fire, which had squadrons alternate between advancing and covering other squadrons.
Different weapons, including sub-machineguns, trench knives and a heavy use of grenades.
Even today infantry uses basically the same tactics when assaulting a fortified position.
Did fighter planes or tanks get better?
They did but neither was all that great against heavy fortifications.
What about gas? The geneva conventions weren't until 1949, if I recall correctly, so why was it less prevalent?
Neither side in WW2 wanted to have the other side retalliate by using gas, so they didn't use it themselves. This was a time when bombers could and did reach the civilian population. Dropping gas canisters on your enemy might have resulted in them dropping gas canisters on your cities. Also, gas has the problem of lingering in an area, poisoning the water etc. It was a desperate attempt to break the stalemate in WW1.