How to attack another Player?
You can only attack Players of rival Guilds (indicated in red on the Map). You can attack enemy Units when they are mining on mining nodes or when they are Reinforcing a location, e.g. resource drop, Outpost or a friendly base. You can also attack their Base directly. To attack just tap on the target and choose "Send Squad". You are then directed to form your attacking Squad from the Units you have currently in your Barracks/Garage/War Factory and can send to attack. Once your attacking Army reaches its destination, it will engage in combat with any enemy Squads in that location. The combat happens automatically and takes a few seconds. Once the combat starts, there is no retreating option - the combat will continue until one side has lost all their Units.
Once the combat has been resolved, you can find a Combat Report in your Inbox. The Combat Report contains details of the combat on how many units each battling side had, how many units each side lost, who won the battle and how many Resources were plundered.
If your attack was successful and you targeted a Base, mining units or an Army contesting an Outpost, your attacking Army will automatically return with as many Resources as the surviving units can haul in their cargo (some units have bigger cargo carrying capacity than others). If you targeted an Outpost, your surviving Squad will now stay and contest the location. You can withdraw such Squad at any time by tapping the location and choosing to Call back.
All your attacking units that got destroyed in the battle are gone for good and you will need to train new ones in your Barracks, Garage, War Factory in order to replenish your Army.
You cannot attack Bases that have an active Shield.