I can only speak directly to the sub-question: No, the Salvation Army is not unique in adopting a military structure as a religious group.

There are several "military orders" within Catholicism. The most prominent today is probably the Knights of Columbus - in the U.S., they're the ones you're most likely to see at a special Mass or in a parade or something. They're a fraternal society, founded in the 19th century, and probably have more in common with the Elks or Shriners than with a military unit. They do tote around swords, have a color corps, and are active on military bases.

More army-like, perhaps, are the groups known as the Knights Hospitaller, or "The Order of the Knights of Saint John of Jerusalem," also known as the Knights of Malta, one branch of which is the "Sovereign Military Order of Malta". They're still a part of the Catholic Church today, and still escort pilgrims on pilgrimages, such as to Lourdes, carrying on a tradition they say goes back to the Crusades, when the Knights of St. John would tend to the injured in the Holy Land.

Hospitallers are considered a lay order (so a religious order, but not one you'd have to be ordained to join), and also a kind of stateless state - the SMOM used to run Malta but were kicked out by Napoleon, yet retained their organization, sovereign status and even issue passports. (Note that SMOM and other Knights Hospitaller don't necessarily share that much of a power structure, and the one has sued one of the others for trademark infringement.)

There's also the Teutonic Knights ("The Order of Teutonic Knights of St. Mary's Hospital in Jerusalem"), which traces itself back to a band of knights who operated (really, re-opened) a hospital in the 1100s, and which absorbed smaller orders like the Livonian Order.

The oldest - and probably most military - of the military orders was the notorious Knights Templar ("The Order of the Poor Knights of the Temple of Jerusalem"), with more groups today claiming descent from them than you can shake a stick at. During the Crusades, they had ranks and titles like "sergeant" and "commander," and fought in battles and won. They had no real links to hospitals or caring for the sick, unlike the Hospitallers or Teutonic Knights. And they lived under the Rule of St. Benedict, taking religious vows that were stricter than the later military orders.

Beyond Catholicism, I have a feeling one of those Early Modern, post-Civil War English groups also had a quasi-military structure... maybe the Diggers? ... but someone with more expertise than me will have to talk about that.

Martial imagery in the Christian tradition goes back to the Bible, of course. And you can talk about the Ecclesia militans [(Church Militant)] (http://i.imgur.com/c8HqyiS.png) of patristic tradition, which refers to the struggle on Earth as opposed to the Church Triumphant in heaven. (The Salvation Army is Protestant, so we can rule out inspiration from the medieval ecclesiastical knighthood orders, hehe.) The Salvation Army's extensive use of the imagery, however, comes out of a general fascination with militancy or militarism in Victorian culture more broadly--a development that was inseparable from, and partially driven by, its appeal within contemporary revival Christianity.

Olive Anderson, "The Growth of Christian Militarism in Mid-Victorian England," traces rising civilian adulation of the military in Britain to the Crimean War. Popular media/propaganda like Hedley Vicars presented soldiers as the "people's army" and worked to make citizens sympathize with the troops. (That specific book even merited YA adaptations.)

Religious missionaries, above all from noncomformist movements (Wesleyan, Baptist...the general Holiness umbrella), saw soldiers as potentially-Christian heroes who accepted their fates with stoicism, not morally bankrupt cast-offs. Their ministry among the troops played an integral role both in "remaking" the military attractive to society, and in viewing their own work as a battleground of sorts. This alignment of Christian nonconformity was only heightened when the army officially allowed soldiers to declare themselves "other Protestant" by religion: it was the place where the Holiness (not "Anglican Etc.") denominations gained recognition.

By the later 19th century, military symbolism pervaded civilian life. The Boy Scouts are the most prominent example today, but a lot of the progressive political/religious movements used military language, too. Stories of "wars and future wars" were best-sellers in Europe. Stuart MacConnell argues that it was in part a response to splintering Christian diversity and, in the U.S., rising immigration. Military rhetoric/structure provided a unifying force for people of multiple backgrounds (he compares it to a 'kinship model' like the DAR). And, of course, a lot of scholars have pointed to a crisis in masculinity, because masculinity is always in crisis: urbanization was making men soft; militarism offered a way to appear TOUGH.

When the Booths brought their particular brand of revivalist religion back across the Atlantic, their first efforts as the Christian Mission actually foundered pretty badly. (Norman Murdoch, Origins of the Salvation Army, is the classic account.) In 1878, they officially rechristened as the Salvation Army with full-blown military symbolism. Lillian Taiz thinks this proved essential to their ultimate success: the Salvation Army's "hallelujah lads and lasses" were overwhelmingly lasses. Military language provided a needed legitimacy to women's socio-religious action.

This plays out in the U.S.-based Women's Christian Temperance Union, which even went so far as to adopt a practice of teaching children marching and other militaristic drills as a way to attract children to the cause of prohibition before the ideology that stronger femininity needed to oppose masculine brutality won the day. They were clearly playing on militarism's cultural popularity as well as cultivating a more 'muscular' image of their organization and movement.

And so, after a decade of failure, the Booths' brand of salvation-for-some evangelical Christianity sharpened into social ministry among the urban poor became both the Church Militant and the Church Triumphant.

The Salvation Army is not unique on this matter. They were part of a cultural mileau in the late 19th century in Britian sometimes refered to as "muscular Evangelical Christianity." This placed emphasis on the spiritual battle and was concerned with trying to win souls in "darkest England." The military symbols were therefore to be seen as part of how they viewed the role of the Christian in society, constantly waging war on sin and evil. The uniform also became a distinctive way of marking them out as members of the Salvation Army. Many modern members of the Salvation Army therefore see the uniform as their own public decleration of their faith.

It's important to remember that the Salvation Army were originally not set up to become a denomination but instead were a group that was concerned with evangelism, temperance and good works. Other groups arose with the same ideas (and uniforms) such as the Church Army (linked with the CoE) but because they did not end up as seperate denominations they are a lot less prominent.

Hopefully that has gone at least some way to answering the question.