Missions are one of the four places one can send their military (the other three being: peacekeeping, sending to attack others, and sending to defend others). These are similar in a way to peacekeeping, as they are neutral to others. They can be accessed from the bottom right of the screen, through the 'bull's eye button'.
Missions are a battle, where you can find out what are the opposing troops are beforehand, and can be set at 5 different difficulties: Easy (no level increase), Medium (+5 level, unlocked at tier [insert if you know]), Hard (+15, unlocked at tier 6), Very hard (+20, unlocked at tier 7), Insane (+40, unlocked at tier 10). The advantages of increasing the level are to get more loot tokens, but keeping the difficulty low, helps mainly get the reward from it/cycle through missions.
There are five rarities of missions (as with most other things): Common, Uncommon, Rare, Epic, Legendary. For a list of missions, sorted into categories, go to List of Missions
Reasons for doing missions over other things
Completing missions are a vital part of the game, and there are many benefits of doing so.
Getting Unit XP (fast):
If you have a strong enough division, you may be able to force your opponents to retreat without launching a single attack. This strategy is called "insta-winning". It can be used to gain XP and level up units quickly, as they do not get injured and can immediately start another mission after finishing one. It can also be used to maximise the profits from the benefits below. However, the downside of insta-winning is that it involves a lot of micromanagement, and you must log into the game and check every less than 5 minutes in order to make it effective.
Some missions, including but not limited to 'Ace of Spades', 'Foreign affairs', and 'Rumble In The ... Never mind' give funds once completed. This is the primary source of getting funds, as (with the exception of coins, which requires real money) it is the most efficient way to get funds. These, once completed, give a significant amount of funds that can be used in other ways.
Outside of spending real money, some missions, including but not limited to 'The Mortar Rain of Abla Nera', 'Dul Kaddir Convoy Interception, and 'The Impenetrable Norrland Wall' give mystery boxes once completed. The mystery boxes have a chance of giving the "+1 Resolve Battle" item, which is useful as it can let you speed up more than one battle (five for premium users) per day. Other things, such as funds, can also be obtained by Mystery Boxes.
Many missions gives out resources upon completion. While not vital, they can help your nation grow much faster. These resources include:
Temperate Terrorists (aka TTs)and the Dul Kaddir Convoy Interception give T72s. These can be discarded for a significant amount of metal. One way (the main way) to get TTs is exhausting the 'Against the Terrorists' card from the 'Union of Nations Card Pack'. This (TTs) then can be farmed by opening Union of Nations Card Packs and it is one of the primary methods of earning metal.
Greatness Points (GP):
A wide range of missions give some GP when completed, such as It's The Least We Can Do or Suppressing the Khev Minosk, and this can be helpful to build up GP, (the bonus category), and over long periods of time, can help increase GP. It also, like most other rewards (for most missions) scales with population, what means it is more effective at Tier 10 than tier 6
Some missions, such as The Impenetrable Norrland Wall (2K22 Tunguska), Venland Airspace Violations (F-35 Lightning) and Dul Kaddir Convoy Interception (10 T-72s), give units when beaten, and can be used to either get metal by discarding, or to get a division. This can be useful as one does not need to build a light armour factory, for example, in order to get 2K22s.T-72s are also basically free version of M1A1s, and can be used to form good mid-tier units.