Hey,
If you have less than X cards in your draw pile then you cannot satisfy the instead clause and the ability takes effect (this is similar to attacking with Jephthah when you have no draw pile). You could (I believe) attempt to discard X cards by discarding all of the cards in your draw pile but the attempt would end up unsuccessful and the ability would still happen (but if you ever want to discard the last few cards of your own draw pile this would be one way to do it
)
Also if your opponent had used Samaritan Water Jar on you so you hard part of your draw pile set aside you would NOT be able to discard Y cards from your draw pile then discard X-Y cards from the new draw pile that appears from your set aside area (where Y is the number of cards in your deck when Assyria Conquers Israel is played).
Tschow,
Tim "Sir Nobody" Maly
