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Entry into the Eastern Cross is only possible (in practical terms) through two Jump Nexi located in the Aquilla and Dunbarton systems. Imperial Doctrine routinely uses choke-points like these to divide space into defensible regions.

The first step in the Duke's Rebellion was to occupy these systems, destroying the Fleet units there, in order to control access to the jump points. Currently the Duke's fleet of auxiliaries is in complete control of these systems and captures any ship which enters from either side, thus preventing word of the Rebellion from spreading. 
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