Answer
One situation in which a budget deficit is justifiable is when there is a war. If there is not a budget deficit during a war, then the government would have to increase taxes. The tax rate would distort the incentives faced by the households who are taxed, and the tax rates would be unfair to the generations who fought in the war.
Work Step by Step
Another situation where a budget deficit is justifiable is when there is a temporary downturn in the economy. In a recession, the government tax revenue is decreased (since most of the tax revenue is based on income). Incomes fall during the recession, so that's why the government tax receipts fall.
The government would want to either raise taxes or cut spending to balance the budget. Doing either of these actions would hurt aggregate demand right when aggregate demand should be improved.
