The HV's and ARBiH's strategic success was a result of a series of improvements to the armies themselves, and crucial breakthroughs made in the ARSK positions that were subsequently exploited by the HV and the ARBiH. The attack was not immediately successful at all points, but seizing key positions led to the collapse of the ARSK command structure and overall defensive capability. The HV capture of Bosansko Grahovo, just before the operation, and the special police's advance to Gračac, made it nearly impossible to defend Knin. In Lika, two guard brigades quickly cut the ARSK-held area which lacked tactical depth and mobile reserve forces, and they isolated pockets of resistance, positioned a mobile force for a decisive northward thrust into the Karlovac Corps area of responsibility (AOR), and pushed ARSK towards Banovina. The defeat of the ARSK at Glina and Petrinja, after a tough defensive, defeated the ARSK Banija Corps as well since its reserve was pinned down by the ARBiH. The RSK relied on the Republika Srpska and Yugoslav militaries as its strategic reserve, but they did not intervene in the battle. The United States also played a role in the operation by directing Croatia to a military consultancy firm, Military Professional Resources Incorporated (MPRI), that signed a Pentagon licensed contract to advise, train and provide intelligence to the Croatian army.