Cadia is strategically situated very close to the Eye of terror, within a narrow strip of stable space called the Cadian Gate. Its inherent stability means that this is the most predictable route for any major chaos incursion in to the imperial teritory, as a battlefleet of any significant size would not be able to safely navigate the other, smaller but much less stable passages.
The Cadian military is very well equipped, armed and armoured to fend off the frequent chaos incursions which vary from small raids to major invasions. The Cadian shock troops are at the forefront of this containment war, reacting rapidly with their mechanised might and bringing their considerable firepower to bear on the invaders. Only rarely, when the forces of chaos are extremely strong do they ask for assistance, particularly from the Legions Astartes. Cadian shock troops are well trained and disciplined, and are highly valued by Imperial Commanders and respected by those Spacemarine forces who have fought by their side.
(Ref: Codex Imperial Guard, 2nd ed; Page 11)