Power in the game reflects your "battle-worthiness", or simply put, how strong you are. It may occur to you that whenever you build or Upgrade something, or train an additional bunch of units in your Command Center, your power value steadily increases. On the other hand, when you lose soldiers from a mission or when attacking other players, power value diminishes as well.

When you get to the point that you'll start to plunder other players' bases, prior to the attack, you'll see the player's basic Command Center details -- including their current power. As a good simple measure, do not attack players that have a huge difference in their power compared to yours (you must at least be stronger than them). Power that is almost equal to yours would result to lots of casualties from the attack that would make both parties suffer. In case there is someone that you want to attack but has a tremendous gap with your current power, you may opt to request for a rally-attack with your stronger corps members.

Ultimately though, you might want to increase your power first before plundering other player bases in the map. Increasing your power can deter other players lower than your power rating from attack your base. A good and effective strategy is to locate a place surrounded by many other player bases, and make sure you have the highest power rating among the cluster. This will be a good preventive measure from being plundered other than casting an Aegis Shield.

To increase your power rating, you may do any or all of the following:

Build/Upgrade buildings and Research Technology

Building Facilities and structures, upgrading them, and researching tech always provide support in your military growth, economy, and Command Center defense -- which in return, will raise your power rating. Each of the buildings in your command center has a small plate indicator in its icon - showing its current level.

(the image above shows particular power increase whenever you reach a certain level of upgrade to the Commando Camp. Not limited to this though, every facility in your command center shares the same feature of increasing your power rating as they get upgraded.)

Quick Tip: Make sure that you have upgraded them to their fullest cap level, before moving on higher tier of upgrades which will cost more and takes longer build times. Additionally, make sure to make good use of the Free Upgrade (UFO icon) when possible. Use it on stuff that counts most. For instance, Resource Collectors usually take just a little time to upgrade and not that costly too -- which makes it a bad choice for the free upgrade.

Train Soldiers

Whenever you train a new batch of units (soldiers, war machines, and even defensive Facilities), you basically add figures to your military, thus increasing your power rating. Not limited to this, when you choose to Upgrade your soldiers ("Promote" the existing ones in the "Parade Ground"), you'll also increase your power as higher level soldiers generally add power to your army more. Additionally, when you try to strengthen your soldiers (available option on any training building), you are basically improving their stats -- which will relatively increase the power rating you'll receive. The same is true when doing a "Special Training" with soldiers level 5 or higher.

(Not limited to training soldiers, building Defense Facilities in your Defense Factory would generally increase your power rating as well. Whenever you do something that particular increases the volume of your army, or strengthen their features [by promoting them or strengthening them through special training], it usually bumps up your total power rating).

Assign Officer/s

You may also assign specific Officers to departments where they could be of use or be effective to gain the additional benefits they impart from being employed in these special roles, as well as increase in your power. Similar to your soldiers, the higher the level of an officer, Rank and Star level, the higher they add to your power rating.

(when you reach level 8 in the game, you'll have access to the Departments Facility. This facility appropriates your officers to certain tasks that they are particularly useful with [doing do will grant you bonuses in resource gathering speed, construction time, etc.]. The game automatically suggests the officers that have plausible skill sets for specific tasks -- therefore you could not simply assign anyone to govern a specific function. Sometimes, you'll have to unlock a special skill of an officer [by promoting], in order to make him/her able run a department.)