The Islamic State in the Sinai posted a video of a former ISIS fighter being executed for selling arms to Hamas.  The group also called on supporters in Gaza to attack Hamas security installations. 

This amounts to a declaration of war by ISIS on Hamas.

Times of Israel:

"[Hamas] uses its smuggled weapons to empower that which was not revealed by God.  It also fights supporters of the Islamic State in Gaza and the Sinai and prevents the migration of these supporters from Gaza to the Sinai," said a speaker in the video, who is referred to as Abu Kazem al-Maqdisi, an Islamic State preacher in the Sinai, originally from Gaza.

Maqdisi calls on viewers to attack the security headquarters and courthouses of Hamas in Gaza, as these are "the pillars of tyranny."

At the end of the video, the narrator declares that a man, named as Musa Abu Zamat, who was once among the ranks of [the] Islamic State, was sentenced to death for "smuggling weapons to the apostates of the Izz a-Din al-Qassam Brigades," referring to Hamas's military wing.

The accused is then shot in the back of the head.

The situation drips with irony.  ISIS wants to go to war with Hamas because the Gazan terrorists aren't radical, violent, or extreme enough for the Sinai terrorists.

Until about a year ago, ISIS in the Sinai was bothersome but not much of a threat to Egyptian security.  All that changed when ISIS began to lose badly on the battlefield.  ISIS fighters began an exodus from Syria and Iraq to the Sinai.  With augmented numbers, ISIS-Sinai has become a deadly terrorist organization, as it proved in late November with an attack on a mosque that killed more than 300 people.

The attack was planned with military precision and carried out with deadly efficiency – a far cry from the hit-and-run attacks on Egyptian security forces that ISIS-Sinai was known for previously.

So why is ISIS going to war against Hamas?  Clearly, the Islamic State sees Gaza as prime recruiting territory, as well as a potential staging area for attacks directly against Israel.  They probably aren't strong enough yet to dislodge Hamas from Gaza.  And it's almost a certainty that Israel would never allow ISIS control in Gaza.

But they can create a bloody mess on Israel's border, creating problems for the Jewish state and throwing the peace process even more into doubt.