They did not wish to strive in Allah’s way with their wealth and their selves; and they said (to the others), “Do not go in the heat”. Say (O Muhammad), “The fire of Hell is fiercer in heat”; if only they could understand.
— Surah at-Tawbah, 9:81
In the 9th year of the Muslim calendar, the Byzantine forces converged on the borders of Syria, planning to mount an invasion of the infant Muslim territories. Rumours of the danger were carried to Mecca by traders. They said the invading army of Heraclius (then-Byzantine emperor) numbered anywhere from 40,000 to 100,000 men. On top of this, several Arab tribes such as the Lakhm and Judham were allies with the Byzantines.
Prophet Muhammad (SAW) heard about it and decided that best course of action was a preemptive strike. He urged affluent Muslims to make significant donations. A number of them who were unable to make donations or volunteer were so disconsolate that Allah (SWT) sent down a revelation exempting them from the obligation of joining the expedition. There were other Muslims who refused to join the expedition despite being in good health and having ample funds. They told the Prophet (SAW) that they would not go out in such intense heat.
Despite these setbacks, the Prophet (SAW) was able to gather a force of 30,000 fighters — the largest Muslim fighting force so far — and marched to Tabuk. The army crossed deserts and baked plains, and had very little water because they camped in lands which once belonged to the Thamud. Eventually their thirst was so great that Allah (SWT) sent down torrents of rain to aid them.
When the Muslim forces reached Tabuk, there were no Byzantines there: by that time, they had abandoned the border towns, so the Prophet (SAW) gave orders for the return march. However, this daring venture made such an impression on the pro-Byzantine tribes in northern Arabia that they shifted their allegiance from Constantinople to Madinah, in the same way that the conquest of Mecca had cleared opposition from those regions.
Where they had previously feared the near impregnable power of the Roman legions, Muslims were now prepared to throw down the gauntlet for the superpower of that era. It is very uncommon in the history of conflict that any conqueror shed so little blood for such a victory.