The need for an army in the modern age

During my thoughts regarding the Territorial Army, and when I took my head out of my hands, I started on another train of thought - the first part was "Why do we need a standing army?". This can be easily answered by the need for full-time trainers - someone has to train the TA; except the second thought creeps in "Why do we need an army at all?".

From first principles what is the need for an army?  We need an army to defend ourselves from attack; we need an army to attack another ourselves. Given our supposed 'civility' we're unlikely to chose to attack, but instead defend. Against what or whom though?

Terrorists seem to be the only ones attacking so we send in the army to attack them. More specifically we send them into the country that is supporting them, remove that country's leaders, and have the populace elect a new set. Which is great provided they also don't support the terrorists; the large flaw in democracy.

In theory having an army descend on you and tear your country apart to kick out a bunch of terrorist-lovin' leaders is unlikely to make a populace elect an exact same set lest they just get more of the same.

So an army is necessary in that sense; more so than a police force.

Circling back to the issue of a standing army though leads me to the current US Presidential race in particular the Republicans and their despising of the welfare state and their insistence on adhering to the will of the "Founding Fathers".

One of the points of the second amendment regarding the right to bear arms was the dislike for the need for a standing army; it also accounted for $683.7 billion in 2010.

So given their stated policies why haven't we heard the Republicans decrying these 'moochers'? I'm sure it's got nothing to do with the military's tendency to vote for them.