Egypt and Syria were briefly merged into a United Arab Republic in the late 50s, essentially a Egyptian takeover of Syria in order to bail out the Syrian Baathists from insurgent communists. It fell apart when Syria resented being too junior of a partner, and they split back apart after 5 years.
That came to mind when I saw jihadis in the Sinai hooking up with ISIS. This time, the insurgents are teaming up.
Interesting times. You can't call ISIS a republic with a straight face, so the title is a bit of stretch, but it's an interesting parallel with the old UAR.