Imperial Army to Imperial Guard

[Wildeybeast]

So, probably a question that has an easy answer, but I don't know it. When and why does the Imperial Army change it's name to the Imperial Guard. I'm assuming it is to represent the fact that they are no longer an army out conquering the galaxy but a defence force desperately trying to protect the Imperium. Is this correct and if so who makes this change and when? Anyone have anything concrete from the fluff on this.

[bfmusashi]

Once upon a time I can recall the IA to IG and IN switch happening during the post HH reforms.

[mjasghar]

not exactly.
the crusade was all about the marines, with them being in charge etc. and army just as support and garrison.
the human element - the army - that remained loyal have been effectively elevated to top rank. It is more usual for crusade fleets now to be led by human generals and warmasters - indeed, i dont think any m40 space marine leader of a battlegroup has used that term, though plent of humans have (slaydo, maracaroth, solon). Now, SM chapters are adjuncts, special forces called in.
The sacrifice made by that Human army is celebrated by turning them into the Guard, as in guarding the Emperor by insinuation.
The idea that the Imperium is no longer expanding so its just a Guard is also implied in the name.

[The AKH]

The Imperial Army was a massive combined-arms force meant to fill in the gaps between and behind the Legiones Astartes, essentially. Each expedition fleet or battlegroup contained ground troops and the starships needed to transport them under unified command. They were also subordinated or in some cases directly under the command of Space Marines; this meant that entire battlegroups turned "traitor" during Horus' rebellion.

After the Heresy, the Imperial Army was separated into the Imperial Guard (Departmento Munitorum) and the Imperial Navy. They are two discrete entities, each operating at the behest of the other, and neither has any commanding authority over the other. This makes it harder for entire formations to desert or betray the Imperium. Even the Warmaster of an Imperial Guard crusade group must still ask a frigate captain nicely if he'd like to have his regiment transported to the next theatre.

[Wildeybeast]

While the Space Marines were at the forefront of the Great Crusade, the IA had a far greater role than you give them credit for. There are hundreds of expedition fleets, and not all of them had Space marines attached. I should perhaps point out that I was looking for actual fluff references to this, rather than speculation. Does anyone know if there is any published fluff? Something in the IG codex maybe?

[Wolfshade]

In the Horus Heresy Collected visions it points to the Imperial Army actively persuing their own campaigns without Astartes support.
There is some background on it in 2nd ed of the Imperial Guard codex that discusses how the Imperial Army was split into Imperial Guard and Navy similiar to the way that Astartes legions were split into 1000 man chapters so that no single body was too powerful.