Much is said about rulers who conquered large swathes of territory, such as the Romans or the Mongolians, yet discussion of Muhammad generally centres on the spread of influence via religion. Just how militaristic was Muhammad and early Islam, and how much military prowess did he or his successors hold?
In 610 Islam doesn't exist. The Arabs are considered so worthless than neither the Romans nor the Persians even bother to invade them. Those superpowers prefer subjugation through taxation.
In 623, the Muslims are so weak that they're about to be either killed or enslaved by pagan tribes in Quraish, Mecca. Those who aren't caught find refuge in Madina or Abyssinia.
By 633, those same people will be able to take on the Persians, the Romans and the warring Arabs tribes headed by people like Tulaiha alAsdi (the quintessential tribal leader: soothsayer, poet, eloquent speaker & brave warrior), Musailamah alKatthab (worst threat to the Muslims. Very peculiar fellow. His fake Qur’anic verses still exist in history books today) and an ex-Christian self-claimed prophetess named Sujaah bint alHarith aTaghlabiyyah. She's a persuasive woman and has the temperament of quirky warmonger. In order to defeat the Muslims, she will do the following:
Early Islamic history is full of these fascinating case studies and varying talents vying for power. My point is, the Muslims would not have survived if they hadn't repeatedly displayed military prowess. Their reputation helped them gain victories when their resources failed them. Look at Battle of Yamaha or Battle of the Chains or Battle of the River or Battle of Yarmuk....These were fluid fighters with an uncanny ability to adapt to changing circumstances.
Khaled bin Waleed gets most of the fame for having fought about 99 battles and never lost, but there are others who deserve recognition. His childhood buddy Ikrimah ibn Abi Jahl, Abu Ubaidah ibn alJarrah, Amr ibn alAas to name a few.
I thought you might be interested in Abu Bakr's letter to Yazid Ibn Abi Sufyan when he sent him to Palestine with a force of about 7,000 soldiers. It gives some insight into how he managed armies from far away:
“I’m appointing you to lead this army to test both you and your mettle. If you do well, I will give you more (duties), and if you don’t, I will remove you. Be conscious of God, because He can see your inside just as He can see your outside (actions)....I have given you Khalid’s job ^1 . Stay away from the arrogance of tribalism and (stay away) from ignorance, for God hates those ways and hates the people who follow them.
When you reach your soldiers, be a good companion to them. Start your relationship with them in a good manner and promise them good things to come. If you deliver a speech to them, then be brief, for indeed, the nature of speech is that part of it makes one forget the other (part).
Improve yourself, and as a result people, the state of your soldiers will improve for you. Pray on time and complete your bow & prostrations and have sincerity in prayer. When messengers from the enemy come to you, be kind with them & generous, but only let a short while pass before you make them leave your encampment, so as to keep them ignorant of it ^2 . And whatever you do, do not let them see you where you are vulnerable, otherwise they will act against you based on the same knowledge that you have about your own army. When they come to you, have them sit with you in the most impressive part of your encampment. Forbid your soldiers from speaking to them, you alone should talk. Do not reveal your secrets to those messengers, otherwise you will hurt yourself and your cause.
When you seek advice, give truthful speech, and as a result, you will be given turthful counsel. Do not hide from your advisors key information that you have, otherwise you will only expose yourself (as an incapable leader). Stay up late at night with your companions and you will receive news and veils will be lifted. Keep many guards, and spread them through out your encampment.
Frequently surprise them with visits, and whoever you find negligent in duty, then punish moderately. Rotate your guards through out the night, and make the first shift longer than the last shift, since it is easier given that it is close to the previous day. Do not be afraid punishing those who deserve it.
Do not spy on your soldiers, nor reveal their secrets to people. Be content with what they show you outwardly. Do not sit in company with fools who waste their time, but sit with people of truth & loyalty. Avoid wrongfully taking from the spoils of war, for it draws poverty closer and victory further. You will find people (non-Muslims) who have confined themselves to monasteries, leave them alone, and leave them to the purpose for which they confine themselves”
Before the advent of Islam, the Arabs kept records of their battles in what is know as Ayam Al-Arab (Days of the Arabs). After Islam, they kept much more detailed records of their battles in something called Books of Maghazi. Contrary to popular opinion, these records were not recorded a couple of hundred years later but rather were studied and examined among the first generations. The reason this was successful was because each generation never stopped evaluating those sources. See Dr. Jonathan Brown's historical analysis here. Source for the letter can be found in Al-Kamil aka The Complete (History) by Ibn Al-Athir (d. 1233, one of several historians who travelled with Saladin's army).