The Knights Templar (Poor Fellow-Soldiers of Christ and of the Temple of Solomon) are an example of a Christian military order, but were dissolved in the 14th century. Are there any currently existing Christian military orders?
The Knights of Malta also known as the Hospitallers of St. John of Jerusalem is a Military Order that still exists.
The most important of all the military orders, both for the extent of its area and for its duration. It is said to have existed before the Crusades and is not extinct at the present time. During this long career it has not always borne the same name. Known as Hospitallers of Jerusalem until 1309, the members were called Knights of Rhodes from 1309 till 1522, and have been called Knights of Malta since 1530.
In times of war, since 1870, the order has been devoted to ambulance service on the field of battle. Catholic Encyclopedia
A short HISTORY OF THE KNIGHTS OF MALTA is an excellent read.
Fra’ Giacomo Dalla Torre del Tempio di Sanguinetto, elected Lieutenant of the Grand Master of the Sovereign Order of Malta yesterday, has sworn oath today(April 30, 2017) in the Church of Santa Maria in Aventino, before the Council Complete of State (the elective body of the Order of Malta) and Archbishop Angelo Becciu, the Pope’s Special Delegate.