You can see your alliance’s attitudes from the Alliance Info tab.
(after clicking the Alliance button at the top of the screen).

Chancellors and Vice Chancellors of an alliance can set other alliances to “Friendly”, “Neutral,” or “Hostile.” 

When you do so, both alliances are notified of the change.

When you set your alliance as Friendly to another alliance, your members no longer have the option of attacking their members.

Setting it to Hostile has no other affect beyond making your intentions clear.