@KhoDepending on view, Husayan ibn Ali was killed and himself beheaded after being killed by Hurr ibn Yazid Riyahi at the
Battle of Karbala. Shia view attests he and his followers were executed and decapitated, where the Sunni view simply makes the claim Husayan was killed by Hurr's spear before being beheaded.
And in a half-sense, Rostam Farrokhzaad was beheaded after the load of him camel was cut, crushing him under his weapons at the final stage of the
battle of al-Qaadisiyysah.
So it's not like it hasn't happened nor has had its precedent during the Rashidun era.
And yes, the Rashidun are already moving into the Ghassanid lands, or already well in it.