Flags

Flags are items dropped by the Wall of Steel. All four are dropped by it, and when you use one, the others will clear out of your inventory. You MUST use one for War Mode to start. Basically, when you use one, you choose a "side" that you take in a Scuffle, either the Corruption (Blue Flag), Crimson (Red Flag), Hallow (Pink Flag) and Confection (Gold Flag). The side you pick determines what mobs will attack you: This does not work in Pre-Hardmode or Hardmode, only War Mode. Defeating the Wall of Steel again will allow you to change sides, but you cannot get Flags in any other way.
 * The monsters of your alt biome (ex. Corruption vs. Crimson) will attack you before any other monsters.
 * The monsters of your opposing biome (ex. Corruption vs. Hallow) will attack you, but tend to the monsters they are fighting first.
 * The monsters of the biome that you chose will fight for you if anything attacks you
 * The monsters from the biome with no relation to you (ex. Confection for Corruption) will simply ignore you.