As you mentioned in the comments, you're looking for something around the 16th to 18th century. The first and most important war in the 17th century was the Thirty Years War.
A war created due to religious fervor. It could've been dramatically been changed by Magic. While the side the British supported could be considered victorious, both sides suffered enourmous losses (both in men and coin). Assuming ONLY the British knew about magic, and no one outside of the government would ever know about it, then here's what Magic could be used for:
Creating an Idol
Every single Faction in the world had a ruler (or sets of) that forced a religion on their citizens. Religion was used as a tool to force men to bend to the Church's or Imam's will and thus faking a god given idol to support the British would win them the war.
How would the British win wars by having an Idol?
Crusaders Mustered under Cross Standarts
Jihadists Mustered under the Scythe
Mongols were underdeveloped in technology (But had fiersome archers). Pagan
Russia (ish) was Orthodox and followed an Arch-Bishop
Latin Americans followed Pagan Sun Gods
North Americans followed Spirits and Statues
The British only had to create a menancing looking structure with some sort of symbol. Quite Possibly a Siege Tower with an animal figure and a cross. The Tower would've needed to be completly sealed, meaning that it had to be damaged to get people in it. It should also have a compartiment for an hidden politic (that could cast the magic without being revealed). Troops would be instructed to stay near the tower. The tower should also be equipped with archer port holes and a cannon or at least a balista. It should be slow, but menancing.
Wounded soldiers would be safe from harm as the Tower would be impenetrable (you had to damage the tower to get in, but the tower self-restores itself, or so it seems). The sheer mass of people that would've heat the tower would make it extremely hard to stay inside, and the Magician would be able to lower the temperature of the tower (quite possibly after the troops are desesperating due to the heat so they rally under religious fervor). Telekineses wouldn't be a good thing to use , at least while showing yourself as that may turn yourself into a target. Having an entity that supports a nation that you can't see will strike fear. Having a man that you can try to kill (and you can succeed in doing that) won't be as impactful, unless he really does have extreme powers.
First War: Unifying Europe
A religious war is a touchy subject. The Holy Roman Empire and allies fighting for the Catholic Church and the Lutherans against it. Now have a structure that can damage your troops, destroy your walls and that you cannot damage? Put a few tales about it being a god's gift to the King of England and you just won the war. Add a few more tales about everyone having to submit to the British and you just got yourself Europe.
Second War: Solidifying the Holy Land
Always a complicated place, but very similar to the first war. Arabs fighting for their perception of god. Do exactly the same thing, and you just conquered the most important parts of the known world.
Expansion: Into the North and into South America
As with the Catholics, making the Orthodox submit to you would be a piece of cake. Add the fact that you can heat up your tower AND MELT THE ICE AND SNOW for bonus points.
The Aztecs would fall easily as they can't actually keep with an indestructible fortressd that is able to destroy their villages. That is, if they don't surrender as soon as they see you repairing the damage the fire pots they throw at you.
The Indians (North Americans)
This would be the actually worst part as they can poison you from the inside, so the turret isn't really a viable option, there is however a solution. Cannons. At this point you rule the world. There's no questioning that. Spending extra coin on a bunch of cannons can bring you the Native Americans to your feet and effectively get you all of the known world under your rule.
Maintaining the Empire
This is actually the difficult part. Your powers aren't really that strong. I mean, while you can create indestructible fortresses, you don't have a lot more powers. Sure you can regulate the weather a bit, but still, that's not all that strong (unless you use it to generate revenue with steam machines, but that would turn your politicians into sweatshop workers, not rulers. Plus you'd already get a lot of revenue from taxes, plus you can just take ownership of what you want). Maintaining this empire would have to be due to fear of striking the wrath of god (Possibly having a religious police like in the Middle East would do the trick). Eventually, unless your rule is extremely appreciated you'll incurr in riots, where you'd have to deploy the tower, which would eventually evolve into a set of tanks (which would force politicians to be soldiers again, which is a downside) and strike fear into rebels.
Your empire would fall once acess to radioactivity and mass poison is achieved by rebels though, as you can't fight biowarfare with just repair powers (unless you fight with hazmat suits (but then it would incur in the major thing that you wouldn't want:)
Never turn your politicians into gods
The whole point of this strategy is to make people believe your troops are protected by someone who can destroy them within a second. If you pose yourself as a god and someone challenges you, you can't forget that your powers are really limited, while you can warm places, stop bullets and such, you are not invincible, and having yourself becoming a target would be the ultimate downfall