There are any number of lessons to be taken from the events of Sept. 11, 2001: the sacrifices of the first responders, the stories of those who were killed, the emergence of terrorism as a critical threat to the United States.

Politically, the lesson cited most regularly is what happened in the immediate aftermath of the attacks. Americans came together, stood as one in support of one another and the nation. “The days after September 11th” has emerged as its own political shorthand for a halcyon moment of comity in the political conversation, a thing to which we might always strive.

But it’s worth noting just how that unity collapsed. Politics is diverse in its scope and inseparable from the politicians who guide it. A moment of tragedy can center the country’s attention on a particular political issue and transcend the people responsible for addressing it. But, eventually, and inevitably, other issues again become important, and the elected officials who have to decide what to do again become the target of approval or disdain.

We can track the unity of the country after 9/11 in the approval ratings of President George W. Bush. No president in the history of Gallup’s approval ratings has seen a higher rating than Bush did right after the attacks. Two weeks afterward, nearly 9 in 10 Democrats and independents approved of the Republican president, with Democratic approval trailing Republicans by 10 percentage points.

By the following September, though? Democratic approval of Bush was under 50 percent, 42 points lower than the approval of Republicans. Part of that was because of Bush’s increased focus on Iraq, beginning in his State of the Union address in January 2002. Part of it had to do with the daily ins and outs of politics.

Right after the 2001 terrorist attacks, The Washington Post and ABC News polled Americans to get a sense of their attitudes about Bush and the attacks. We found that, among other things, Americans were broadly supportive of engaging in military conflict, though those attitudes waned as the possible side effects of conflict were raised.

About two-thirds of Americans figured war was likely. Nearly everybody — 98 percent of respondents — said they’d be willing to endure longer security lines at airports to improve security.

The next month, the United States began striking targets in Afghanistan. That action had wide support.

Americans were even generally supportive of sending ground troops into Afghanistan (76 percent agreed) and expanding the conflict to other countries that might be harboring terrorists (87 percent).

By January 2002, though, the economy was in recession and the unemployment rate was spiking. More than half of Americans said that the economy was a bigger concern than the war on terrorism.

A year after 9/11, Americans were still more worried about the economy than about terrorism. Bush’s approval ratings had slipped among Democrats significantly.

Looking back at the attacks, two-thirds of Americans who thought that 9/11 had changed America thought it had changed for the better. A third — fairly uniformly across party identification — said that America had changed for the worse.

At that point, about three-quarters of Americans still told Gallup that they thought Americans were generally united in their agreement on the values that were important to the country. After 2002, though, that agreement tanked.

By January 2003, Bush’s approval rating among Democrats and independents had dropped significantly.

Part of that retreat reflected concern about Bush’s push for military action in Iraq, which was supported by less than half of Democrats and only slightly more than half of the country.

Fewer than 4 in 10 Americans thought that Bush had made his case for the war at that point. Most thought that the risk of going into Iraq too quickly was higher than the risk of waiting too long — including 71 percent of Democrats and 58 percent of independents.

The declining support was also linked to poor reviews for Bush’s handling of the economy. Less than half the country thought that he was handling the economy well — a lower percentage than thought he was dealing with Iraq appropriately.

Half of Democrats thought the economy had gotten worse, and twice as many Americans overall said they thought it was getting worse rather than getting better.

When the war in Iraq began, approval for Bush ticked upward slightly. That said, only 43 percent of Democrats thought it was right to strike Iraq when we did vs. 88 percent of Republicans. Nearly 8 in 10 Republicans thought the country’s vital interests were at stake; 57 percent of Democrats agreed. There was general agreement though: The war would be measured in weeks or months, not in years.

By January 2004, the war was ongoing. Fifty-three percent of independents approved of Bush’s handling of it; only 23 percent of Democrats did. By a 58-to-39 percent margin, Americans thought that the economy was the more important issue to deal with, and less than half of independents and only 1 in 5 Democrats approved of how Bush was dealing in that regard.

In our poll that month, 41 percent of respondents, including two-thirds of Democrats, said the Iraq War wasn’t worth fighting.

The unity around Bush’s presidency had collapsed — and so had that sense of post-9/11 political unity, a victim, as always, of day-to-day politics.