I'm about to do something I generally loathe: divide opinions on an issue (in this case foreign policy) into several types and categorize certain politicians by type. So I'll state at the outset that these categories are intended to be very fluid. Most of us are somewhere in between, and even vary depending on the particular situation at hand.
That being said, it can be useful to describe certain levels of comfort with the use of American military power. Although they do not necessarily align themselves with political ideology, I'll start with the one that seems farthest to the left and proceed to the right.
Level 1: minimal.
This strain of thinking says that American military force should be used either not at all (conscientious objector status) or perhaps only in cases of strict self-defense, to defend against an invasion from a foreign power. To clearly differentiate it from Level 2, Level 1 thinking would have opposed the war in Afghanistan - although bin Laden attacked us, the Taliban government had not declared war on the United States. Dennis Kucinich would be an example of a politician whose views lie primarily in this category.
Level 2: retaliation/deterrence.
In this category, use of American military force is again justified only by self-defense, but self-defense is viewed more broadly. In particular, this set of opinions is willing to use force to retaliate against an attack, or to deter an imminent future attack. In contrast to Level 1, Level 2 supporters would likely have favored the war in Afghanistan as a combination of retaliation and deterrence. Most people in this category would have opposed the Iraq war, as Saddam did not present an imminent threat to the United States (though BushCo's lies may have confused a few). Politicians in this category include Barack Obama, though some of his actions are closer to Level 3.
Level 3: humanitarian.
People in this category are more comfortable with using a limited amount of American military force to improve (or attempt to improve) the condition of the world. In particular, this strain of thought, is generally supportive of air strikes, and possibly a small number of special forces, to prevent humanitarian crises. Most people in this category also believe that it is far preferable to have a majority of the international community behind such an effort. Level 3 opinions would have generally supported the American military interventions in Bosnia and Kosovo during the 1990s, and would have regretted the decision not to go into Rwanda. The majority of Level 3 people would have supported the Iraq war at the time, as it was sold as an attempt to make the world a better place by getting rid of the dictatorial Saddam. However, many of them have now changed their minds on Iraq. I feel comfortable placing Hillary Clinton in this category, as well as Samantha Power. John Kerry is arguably also a member, though he shades towards level 2. Barack Obama's actions in Libya are also good examples of Level 3 behavior. Some people use the word "neoliberal" to describe this brand of foreign policy.
Level 4: dominance.
This is the neoconservative school, willing to use American military power for the express purpose of maintaining American hegemony. People in this category would be willing to launch a preemptive war against any country they perceive as challenging American dominance of a particular region. This is usually limited to regional powers like Iran rather than nuclear powers like Russia, as even the most radical members of this group are leery of nuclear war. This group would have strongly supported the Iraq war, as it gave the United States control of Iraq's oil reserves, and they would not care that the stated justifications turned out to be false. Many of them believe that a preemptive war with Iran is a good idea. Dick Cheney is the prototype.
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Although these categories are by no means perfect, I think this scale can be useful for making sense of some of the current foreign policy debate as we head into 2016. It is useful to have ways of describing the shades of gray in between Dennis Kucinich and Dick Cheney.
Within the Democratic primary, there is a choice between a politician who is a pretty good example of Level 2 (Sanders) and a politician who is a pretty good example of Level 3 (Clinton). Democrats by and large are split between these two levels (though with a subset of Level 1 supporters - maybe 15-20% of the party, judging by Afghanistan numbers), and I think the Sanders/Clinton choice is a fairly typical one to have within the party, representing a real difference of opinion. On this issue it is similar to the Obama/Clinton choice of 2008.
That being said, there will also be a clear contrast in the general election, regardless of the identity of the Democratic nominee. The Democratic nominee will be Level 2 or Level 3 - perhaps too comfortable with military force for one's personal taste. However, the Republican candidate, whoever that is, will be Level 4. There is a real difference between "neoliberal" foreign policy (level 3) and neoconservative foreign policy (level 4). A full-scale regional war somewhere, involving a large number of ground troops, is far more likely if the Republican candidate wins in 2016.