You must be in a guild to enter the Great War. The 'Great War' button is in the same spot as the 'Headquarters' button and will be grayed out until the Great War begins.
Each time you enter take note of the color and name that you will be fighting for as it changes with each Great War. This time (below) you are Blue/Brunhild and you have 10 AP to use up . Note* each (battle/square) you fight in uses up 1 AP. In the center of your blue 'occupied' square of land is your Fort. And at the very top of the picture you can see a square containing 2 'dots' in it... this is the nearest Turret so you'll want to try to capture it for points before a rival does.
Chose your square carefully to be certain another on your team is not fighting in that same area. Call your coordinates out as you enter to let others know. This square is -12 -10. If it was not bordering the rest of your blue captured land then the 'attack' button would not be available.
Now that you've captured a few squares note that your score has increased. Blue now has 182.
Here blue has invaded to the heart of the yellow's and captured their Fort. The score now has increased signifcantly.
All resources are shared within the guild and your Guild Leaders would be wise to use them to fortify their captured Forts and Turrets to make them harder to invade by an opposing team before the Great War ends.
During the Great war you can set defenders to protect your lands from invaders (Other players). Whilst defending, a unit cannot partake in invading other lands unless set as no longer defending.
To set units as defenders, click the icon circled in red in the Unit List Menu.
When a defending unit kills an invading unit, the defending unit gains a point. The more points a unit has, the higher in the defense rankings he/she will be. A successful defense also generates contribution points for the player.
Note you must be assigned a faction for your units to defend. You are assigned a faction when you join the great war.