Seems to be conflicting information currently. One the one hand,
a. In the HUD upper right, I get a blue smiley and the mouse-over tells me I am allied. In the Foreign Relations, the arrow on the scale is to the right and is labeled "Ally".
And on the other hand,
b. I receive the message that my Alliance Treaty has expired, and on the right side of the Foreign Relations register, the Alliance is not listed as amongst those currently existing.
I have diplomatic victory enabled, and in the kingdom report register, alliances are listed as valid (currently 2/4).
Perhaps this is a bug related to the following (simply my conjecture) -- During the same turn, a player successfully creates an Alliance Treaty with two (or more) parties who, in that turn, are each separately at war with one another. I am pretty sure I did this.