The concept of the IG as part of a futuristic fighting force may seem a little strange to our modern eyes. The IG are often portrayed as basic in comparison with current militaries, having more in common with the warfare, tactics and gear of World War I than a future combat force. However, like all things in 40K it is not always as simple as it first seems, and we can't compare our world to the future galaxy spanning might of the Imperium or their many powerful factions such as the Adeptus Mechanicus (note to self: Enemy tech suppression and weird views of genetic management of populations, and form of 'practical' and 'situational' eugenics).
In short the IG is not the Imperium's main fighting force when dealing with rogue planets and aliens - that is down to the Navy. This is an important distinction as the IG is not used in the same way as out modern millilitres. Primarily the IG is a peace keeping force used for Imperial nation building. They are deployed to ensure a stable environment for the Imperium to put into action various programs to fully convert the world over to the Imperial way. Eventual the IG peace keepers change roles once the world is stable and become Arbites and PDF, and after a few generations are fully integrated into the gene and meme pool.
Another aspect of the institution of the IG is one of population control and disbursement. The excess population of super massive and loyal Hive Worlds are routinely skimmed off for service in the IG. These conscripts are shipped to a world that needs to be stabilised. World population losses through the conflicts in unstable areas are literally replaced by IG who will later settle on the peaceful world (should that come to pass) and who know how to live proper hive life. The traitors are systematically replaced by loyal IG.
It should be noted that even though the IG is a peace keeping force is does not mean that they have not been involved in epic ground battles. Though most full scale 40K ground battles are over in seconds, thanks to the Navy's WSDMs (weapons of super massive destruction) which can lay waste to an entire worlds from orbit, there are occasions when the Navy holds its fire even when they have orbital superiority. These scenarios often revolve around large IG presence on a world and the possibility that the IG could win a good old fashioned counterinsurgency/ ground war. The Navy will help out, and any positions overrun will disappear in a flash of brilliant sunfire.
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