Edit 2: This is sort of an open ended title I guess, so maybe we can come up with other seemingly obvious things they should have done.
In that case, why didn't the Borg send 150+ cubes into Federation space, one for each member world? Starfleet would never have the numbers possible to defend against each one.
Commander Austin Harris, First Officer, Deep Space Nine (by FltCpt. Bossco)
8.01 - Darkest Before Dawn
(Chapter 8 added, 12/09/2015)