Albert Barnes Commentary 1 Samuel 15:2

"Thus saith Jehovah of hosts, I have marked that which Amalek did to Israel, how he set himself against him in the way, when he came up out of Egypt." — 1 Samuel 15:2 (ASV)

It appears from the cross-references (see 1 Samuel 14:48) that this expedition against Amalek was not made without fresh provocation. They likely made an incursion into the southern region, similar to the one described in 1 Samuel 30:1, at a time when they thought the Israelites were weakened by their conflicts with the Philistines.