While they all have their purposes, the Navy is often considered the most important. The Army, at least in the US, is quite the unwieldy instrument. In fact, if push came to shove, there are only two units of significant strength that can deploy rapidly - the 75th Rangers and the 82nd Airborne, both of which are light infantry units and consist of less than 20,000 men.
People often believe that the Air Force is the most important, but this is generally false. While the Air Force certainly has the potential to deliver absolutely staggering firepower, likely more than the other three branches combined, in modern warfare this is not terribly useful in and of itself. Sure you can bomb buildings, take out command and control elements, destroy infrastructure and generally cause havock - these things are important but they do not win wars. More importantly, in our current conflicts, often the airforce is utterly incapable of applying more than a tiny fraction of its firepower for fear of friendly or civillian casualties. As such, the Air Force is generally seen as a force multiplier - the simple fact that any infantry company has the capacity to deliver 500+ pounds of precision guided ordinance when they run into trouble is not a power to be underestimated.
I would say, that if one looks at the overall purpose of a military, the Navy is the most important. The reason is simple - the navy is often the closest element to any hotspots that might pop up, and just as importantly a single carrier battle group has more firepower available than the entire armed force of most nations. Every battle group has the capacity to wage a fairly heated war in and of itself. No other branch is so supresmely suited to project overwhelming force at the drop of a hat, and the mere threat of such a body provides a healthy deterrent and helps facilitate negotiations. Perhaps just as important is the fact that the Marines are technically part of the Department of the Navy, meaning one has the capacity to deliver a sizable force of men and (most light) armor all while having a wide variety of force multipliers in the form of Aircraft and guided missiles.
People often believe that the Air Force is the most important, but this is generally false. While the Air Force certainly has the potential to deliver absolutely staggering firepower, likely more than the other three branches combined, in modern warfare this is not terribly useful in and of itself. Sure you can bomb buildings, take out command and control elements, destroy infrastructure and generally cause havock - these things are important but they do not win wars. More importantly, in our current conflicts, often the airforce is utterly incapable of applying more than a tiny fraction of its firepower for fear of friendly or civillian casualties. As such, the Air Force is generally seen as a force multiplier - the simple fact that any infantry company has the capacity to deliver 500+ pounds of precision guided ordinance when they run into trouble is not a power to be underestimated.
I would say, that if one looks at the overall purpose of a military, the Navy is the most important. The reason is simple - the navy is often the closest element to any hotspots that might pop up, and just as importantly a single carrier battle group has more firepower available than the entire armed force of most nations. Every battle group has the capacity to wage a fairly heated war in and of itself. No other branch is so supresmely suited to project overwhelming force at the drop of a hat, and the mere threat of such a body provides a healthy deterrent and helps facilitate negotiations. Perhaps just as important is the fact that the Marines are technically part of the Department of the Navy, meaning one has the capacity to deliver a sizable force of men and (most light) armor all while having a wide variety of force multipliers in the form of Aircraft and guided missiles.
