In Part One of As We Go Marching, John T. Flynn describes the rise of fascism in Italy. The following passages imply the need for two political parties.
A party on the left:
Thus in Italy, while individuals murmured against the growing debt, they continued as members of cities or villages or farming districts to fight valiantly for their share of the spendings. The people as beneficiaries were always more powerful than the same people as citizens.
A party on the right:
Some form of spending must be found that will command the support of the conservative groups. Political leaders, embarrassed by their subsidies to the poor, soon learned that one of the easiest ways to spend money is on military establishments and armaments, because it commands the support of the groups most opposed to spending.
And with those two satisfied, the state can expand with the support of the vast majority. At least until it collapses.
Note: Later in the chapter Flynn captures the truth — a truth proved once again as the Obama administration continues the concept of perpetual war for perpetual peace — with this nugget:
Among all the means for producing government-created income none is so successful as militarism.