"The question is, how far do we go to protect ourselves and our allies before we end up becoming an Empire?"

I don't think fear of being an empire should be limit to the degree in which we protect ourselves.
America is actually called a kind of Empire- America is a superpower and that superpower is involved in world affairs.

But America isn't an empire that can give orders to other countries- America [it's President, it's Congress, and/or it's People] doesn't function as Emperor issuing decrees.
Our allies are our allies and not in anyway are they sub servant States.
When Turkey [not merely an ally, but a member of NATO] can deny access to our military- and thereby weaken our military effort in Iraq. That is one obvious example among many that demonstrate how though America is called an empire, it's more of a federation or alliance of nations than an empire.
Turkey had it's reasons for opposing this and these simply ended up trumping our interests.

Now, it could be argued that any wise emperor will function in a very similar manner as America does- well I don't have a problem with wise emperors- as long as they are wise and not merely said to be wise.

So as long as this "America empire" isn't ordering other nations around, I don't see the problem with it being "this kind of empire".
Nor would I have any problem with the EU or China being "this kind of imperial power"- and all they have to do if they want to do this, is demonstrate this kind of leadership.

In an egalitarian society a leader is a leader by doing the work, and asking others to join him. In other words if a road need improvement, he/she repairs the road and ask others to help him/her. So if other countries want to step up and lead [by doing the work], how many Americans are going to have any kind problem with this?
The problem isn't America being some kind of imperial power, it's that lack of other countries functioning like the American imperial power. Though obviously these countries will eagerly take the benefits, they are as a rule less keen to do the work.

The next aspect is what is actually protecting ourselves, rather than what only appears to be protecting ourselves.

This is more complicated. But if our action in Iraq results in Iraq being a significantly "better nation", and our costs of doing this weren't too high, than that would fit in the category of "protecting ourselves".
In other words, global security is what we are actually talking about. And global security has been something the US military effort has been mostly concerned with since WW II [with growing awareness that isolationism wasn't a viable way to actually protect our security].