So, I have this crazy idea for a character build for my DM's homebrew campaign. However, even thought we are using 5e rules.. for whatever reason the class I want to play is being made up from the creator of the game. Where generally speaking, that a majority of what "he" perceives a druid to be like and act like goes against what it states in the 5e handbook. 

The setting: Around a 1000 years ago, a great calamity struck the world creating an apocalypse wiping out alot of the continents and life through out the world. Goodly races now live with evil races.. and there is a constant struggle for whats good and evil. There are evil forces led by Gorgon's and other world creatures that are trying to wreck havoc. Where my small party lives we reside in a very dense like jungle with one city of mixed races. There is a military force present that actively engages these evil creatures to hold them abay. 

My background, I'm a 354 yr old woodland elf. Who has spent the greater part of his life traveling and studying nature and animals of different regions. And yes my class is a druid. This was the case of up to the last ten years of his life. Where I have been acting as a forward scout for the military. Due to my background with nature and my ability to read trails. This was a voluntary thing to make some living money and help fund my constant search of new and intriguing things to study in nature.. however. I fell off track since my services as a forward scout was so renowned. 

However, my DM states that a druid can't be a soldier. Because there only cause is that of protecting nature. 

Can anybody help me make this background fit... I really love this idea being ex-military myself

You choose to be a scout for the army, not because of the fight between good and evil but to steer those who where destined to clash in a direction to mitigate the damage to the planet. You knew you couldn't stop them, but you could protect nature by reducing the harm it faced...