Advance Wars: Days of Ruin, released as Advance Wars: Dark Conflict in Europe and Australia, is a turn-based strategy video game for the Nintendo DS handheld game console. It is the fourth installment in the Advance Wars series after Advance Wars: Dual Strike and was released in 2008 for North America on January 21; in Europe on January 25; and in Australia on February 21. A Japanese release was planned under the title of Famicom Wars DS: Lost Light, but was canceled after a series of delays. It finally became available in Japan through Club Nintendo in October 2013 as a downloadable platinum status reward for the Nintendo 3DS family of systems.

Days of Ruin features a grim and more serious tone and atmosphere in contrast to the previous light-hearted installments, with a new storyline, characters and setting independent of the previous games. Set amidst a post-apocalyptic world, the story focuses on the Rubinelle 12th Battalion, one of the surviving remnants of the military of the country of Rubinelle, which had been locked in a century-long war with its rival, Lazuria, prior to a devastating global meteor shower. In the aftermath, the Battalion devotes itself to assisting survivors and restoring order, despite the shattered nations renewing their war against each other and an incurable disease ravaging both sides. Meanwhile, a mysterious faction with unknown motives takes advantage of the destruction and pushes both sides deeper into conflict from behind the scenes.

The primary objective in Days of Ruin is for the player to use their military force to defeat the enemy's force. Except for certain single player missions with special objectives, there are two ways to defeat an opponent: destroy all of the enemy's units or capture their headquarters. The battle system is turn-based; two-to-four forces, each headed by a commanding officer (CO), take turns building and commanding units on grid-based maps. Every turn, units, which consist of ground, sea, and air units, can move across the different types of terrain and attack enemy units or perform other actions, such as submerging a submarine or resupplying friendly units. Many factors can affect the battle, such as fog of war, a phenomenon that prevents players from seeing enemy units other than those in the visual range of their own units; various weather conditions; and CO powers.

The entire cast from the previous games has been replaced with new characters. CO powers have been toned down, and no longer affect all units or the whole map. Tag powers from Dual Strike, which allowed players to move twice in one turn, have been removed. Players gain CO powers much later in the campaign than in previous games, and they have a much less significant role in overall gameplay.

At the HQ or any unit-producing property, COs can join with a specific unit, and automatically promote that unit to Vet level, but at the cost of half of that unit's value. The CO's unit confers an advantage on friendly units within a certain range, the "CO zone". These effects are generally minor advantages such as attack or defense boosts. CO effects are constant and, unlike previous games, only benefit units within the CO zone.

As damage is dealt by units within the CO zone, the CO's power meter fills slightly, at a rate of one bar per five hit points worth of damage inflicted. As the CO power meter is filled, the CO zone grows larger. When the meter is full, the CO can activate his or her power which has an effect on the whole battlefield, such as repairing allied units, damaging enemy units, or temporarily altering weather conditions. In addition, the offensive and defensive boosts that normally apply only in the CO zone are applied to all allied units on the whole battlefield for the turn that the CO power is active. If the CO unit is destroyed, the CO meter empties and the CO returns to the HQ, where they can be assigned to another unit.

Units can now level up in battle, increasing their capabilities. Units increase their level once for each enemy unit that they destroy. The level of each unit is identified as "I", "II", or "Vet", with "Vet" being the highest level. While units with higher levels are more powerful than new units, the power increase is slight. Unit experience is not persistent, and the player begins each mission with new units. Not only does the attack increase with each level, but the defense of the unit also increases.

Unit prices have been readjusted and some units renamed, as well as new ones introduced. New land units include the Bike, a highly mobile infantry unit that can capture properties; the Flare, an armored vehicle unit that can reveal areas affected by the Fog of War; the Anti-Tank, an indirect-fire unit strong against tanks with the ability to counter-attack during direct attacks; and the War Tank (formerly the Mega Tank), the strongest ground unit in the game. New air units include the Duster, a weaker, lower-cost interceptor that can strike ground units; and the Seaplane, which is produced by Carriers and can attack any unit, but has limited ammo and fuel reserves. Additionally, there is a single new sea unit, the Gunboat, which is armed with a missile salvo that must be resupplied after each use, and can transport one infantry/mech unit. Also, the Battleship now has the ability to move and fire in the same turn, making it the only indirect-combat unit able to do so.