All in all they need to be in a real "no win" situation
I find that blockading most of their planets will cause them to surrender.
Also by beating them in *many* combats so their populations are rioting and angry they surrender more easily.
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The Unofficial Space Empires 4 Mod Utility V1.6
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Download as a .ZIP <
HERE>
Download as a .RAR <
HERE>
Download as a .ACE <
HERE>
Check a Screenshot of it<
HERE>