President Barack Obama makes a statement on Libya, Friday, March 18, 2011, in the East Room of the White House in Washington. (AP Photo)

Obama has been very careful in his public pronouncements about Libya to emphasize the reluctance with which he authorized the use of force and the limited scope of U.S involvement. “This is not an outcome that the United States or any of our partners sought,” Obama said in a statement on Saturday. “We will not -- I repeat -- we will not deploy any U.S. troops on the ground.”

A look at polling on the last three major conflicts in which the U.S. has become involved suggests that Obama’s best political hope is for a short engagement in which the general perception is that America won with the help of a broad coalition of international partners.

Let’s break down the numbers starting with the ongoing war in Afghanistan.

The most recent Washington Post/ABC News poll on Afghanistan showed just 31 percent believe the war is worth fighting while 64 percent believe it isn’t.

Those are the lowest support numbers since the Post began testing Afghanistan in February 2007 and reflect a long term negative trend line when it comes to the American public’s view on the war. In fact, only once since July 2009 have a majority of people tested in Post/ABC polling said the war is worth fighting; that moment came in mid-December 2009, less two weeks after President Obama delivered a speech at West Point seeking to define that mission and rally the country behind it.

The poll numbers on the Iraq war are strikingly similar. After a brief period in 2003 when support for the war ran in the 60s and 70s, there has been a steady decline in the numbers.

Not since September 2004 have a majority of those tested in Post/ABC polling said the war in Iraq is worth fighting. And, in 21 of the last 22 Post/ABC surveys testing Iraq those who say it is worth fighting has been in the 30s — including a September 2010 survey where 34 percent offered their support for the war.

In stark contrast to those two wars, the first Gulf war was, politically at least, the best guide post for what President Obama has to hope happens in Libya.

Public support for the war started high and stayed there; in mid January 1991, 76 percent of people approved of the U.S. going to war in Iraq and by late February 1991 that support number had soared to 84 percent.

The secret to that political success? The war was short -- military actions lasted less than a month — and the U.S. was widely perceived to be at the head of a broad international coalition that soundly defeated Iraqi dictator Saddam Hussein.

Not so in Afghanistan and the second Iraq war; each conflict has been drawn out over years and the international coalition built in advance of those two wars was not only smaller but has also faded considerably since military action began.

Given that history, it’s no surprise that President Obama is focusing almost entirely on the planned brevity of the U.S.’s military involvement and the near-unanimity of the international community in support of the actions taken against Libya.

“Our military action is in support of an international mandate,” Obama said today during a press conference with the president of Chile; later he highlighted the “powerful international consensus” behind the attacks.

Obama and his political team have to hope that his rhetoric on the war — that it is necessary and will be short — matches the reality on the ground. The longer the conflict goes on, the more likely opposition to it will grow and the bigger political problem it could be for the White House moving forward.