From the "Army" page you can manage the movements of your army, as well as the numbers of each unit within each army. Armies 1 to 3 can be sent to attack, or kept at home to defend, whilst the "Home" army represents any units stationed within your kingdom. These will defend your kingdom if attack, but cannot be sent to attack another kingdom without being transfered into another army.
Your armies cannot be sent to attack a kingdom that is too much smaller than you, or a player who is either protected (has recently joined the game), or is in holiday mode.
Once you have assigned an army a target, it will march to its destination over the period of several ticks. At any point during this march you may recall it to your kingdom. Once it has reached a target kingdom, you may Attack.