Space Marines do not always fight alone and often find themselves going into battle alongside other Imperial forces. The mass ranks of the Imperial Guard form the major portion of any allied contingent and include huge tank divisions as well as enormous numbers of foot troops. Other Imperial organisations can and do supply warriors and war machines to the war effort. These include the mighty titan war machines of the Adeptus Mechanicus, the warrior nuns of the Adepta Sororitas and a host of other units and individuals.

When an army of the Imperium goes to war it is standard practice for the Imperial Commander or general to choose a symbol to act as a campaign badge in accordance with the regulations set down in the Codex. This "field sign" is known as the Army Badge and serves to identify all the troops and equipment that are operating under his command and helps them recognise friendly units on the battlefield. The Army Badge is painted onto Space Marine armour and Imperial Guard tank alike. Cloth patches may be sewn onto Imperial Guard fatigues. Huge versions of the badge will be stencilled onto the gigantic armour plates of Titans. Each time the badge is applied the recipient receives a blessing from the Company Chaplain if a Space Marine or from an attached member of the Imperial Ecclesiarchy if serving in one of the other Imperial forces.

The design of an Army Badge is deliberately quite straightforward. Usually it consists of a simple geometric shape overlaid with a motif or it might be a simple stencilled icon. The Army Badge must be instantly recognisable if it is to function as a battlefield identification. Space Marines generally apply the Army Badge to their right leg greave.

The Army Badge is only used for the duration of a single campaign. Once this is over the various forces that comprise the Imperial Army are dispersed and the Army Badge is either removed or replaced. If a squad, regiment or indeed an individual warrior has performed exceptionally in the campaign, the Army Badge may be incorporated onto their armour, uniform or heraldry as a permanent honour. This is particularly true of banners and standards and many of the lavish designs seen on Chapter, Company and personality banners are, in part, derived from the Army Badges of previous campaigns.