Trying to clear a Heavily Defended Precursor Anomaly with one of my fleets

Note the Xeloxi banner associated to the Precursor Fleet. When I try to attack at range, a message pops up saying "you are not at war".

Won't let me attack, since we are not at war. When I try to move on to the hex with my fleet to engage directly, I get a dialog that says Do you wish to declare war on the Xeloxi.
Trying the direct approach below

Are the Xeloxi supposed to be able to control Precursor Dreadnoughts?