Why didn't they? Couldn't a team of Spartan-II's escorting some naval personnel board a Covie Battlecruiser and capture it then allow the naval personnel to take control of it? Rinse and repeat.
I mean we all know that UNSC ships are garbage when facing Covenant ships. So they should've just captured Covie ships to level the playing field.
Would've been pretty effective in my opinion.
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Ok... If you have read the books you would know that the pillar of autumn was refitted to board a cruiser with all the available spartan 2's. Capture the bridge before it self-destructed, get the nav data. Go to the home-planet, capture a so-called prophet (leader) and force a truce. Now... let me check this: they rebuild 1 single ship (fitted it with meters of titanium armor) AND put 30 spartan 2's inside it just to capture 1 cruiser. Yeah seems legit that they didn't try it before... they didn't try it because it was impossible. They were planning on capturing a cruiser, they just never had the means to do it (PoA could sustain extreme structural damage before falling apart and spartans could get to the bridge before well... dying) Ohw and ofcourse the entire plan failed when reach got attacked.