Have to go with World War 1.
19th century military tactics meet 20th century technology! I love researching the technological breakthroughs and (often hilariously bizarre) experimental weaponry and gear that resulted from this.
German A7V tank, anyone? http://historywarsweapons.com/wp-content/uploads/image/A7V1.jpg
This baby was nicknamed the "Rolling Fortress" by the British. It was so slow it would usually be found lagging way behind the infantry it was supposed to protect, and was quite prone to getting stuck on almost any kind of terrain. It was the only German WW1 tank to make it into production, and German tank divisions relied almost entirely on capturing British Mark IV and V tanks.
Mark V tank: http://upload.wikimedia.org/wikipedia/commons/d/db/British_Mark_V-star_Tank.jpg
And just for the hell of it, here's a few pics of the magnificent (but never mass-produced) Tsar tank of Russia!
http://upload.wikimedia.org/wikipedia/commons/4/4b/Tsar_tank.jpg
http://4.bp.blogspot.com/_Bu2MvID6kBQ/TRTxF6UtWFI/AAAAAAAAA0E/coNcaOeJb8I/s1600/tsartank1.gif