The concept of a balanced budget in government finances is intimately tied together to the concept of what is government good for. If you accept, a priori, that you are providing services to the population of today that cannot be accomplished as well privately and you are building things for tomorrow that, again, cannot be better provided by the private sector, you agree to the same sorts of calculations of national interest that families do around their kitchen table when they plan on buying a house or taking on other major expenditures.
Some things are just unaffordable, some things aren't worth doing even if you have the money, and some things are worth going into debt over. To be a fetishist over a balanced budget means that you think that there is nothing so important that it isn't worth going into debt over.
Now, as a libertarian, the list of things that I think the private sector can't do better than government is a lot closer to zero than your everyday average centrist. But it's not zero. Financing a war is probably the most common and biggest effort that is a justifiable reason to run a deficit. Military related expenditures like GPS satellites are also on the list.
Expansion expenses, such as purchasing from sovereigns who would not sell to any but a national entity is another legitimate expenditure. I don't find the Louisiana, Gadsen, or Alaska purchases to be illegitimate and certainly they have paid off over the years. Expenses undertaken to establish law and legitimate security operations in new territory also fit the bill.
So even in a libertarian world view, it's legitimate to go into debt. The expenditure must be for the survival of the country or an investment that cannot be done privately and will benefit the general public over the long haul.