Just a year ago, many greens were bruised, battered, and demoralized after suffering the defeat of decade-long effort to pass comprehensive climate legislation. There was a good deal of soul searching in offices of environmental NGOs. Then, sometime last summer, it was as if all the energy changed, and suddenly a broad range of environmental organizations pivoted, found their voices again, and collectively turned their energies on sinking the pipeline.

As activists savor the victory, here’s a couple of important lessons to keep in mind.

Play Politics. Greens have a reputation for being too nice and not wanting to ruffle the feathers of political allies, which usually means elected Democrats. On the Keystone fight, 350.org founder Bill McKibben ran a different playbook. From the start, he framed the fight as a crucial test of President Obama’s commitment to addressing climate change. On the first day of a two-week-long civil disobedience protest in front of the White House in August, McKibben, NASA scientist James Hansen and others wore their Obama 08 pins as they were hauled off to jail. The message was clear and powerful: You can’t count on our vote again if you approve this pipeline. The litmus test worked. Making Keystone XL a make-or-break issue for the president attracted the interest of the horse race-obsessed capital press corps. Suddenly, a local issue had become a national one, something that threatened to cost a vulnerable president a core constituency. The takeaway: politicize, politicize, politicize.

Just Say No. For me, as an armchair environmental movement chronicler, one of the most interesting characteristics of the Keystone XL opposition was its unanimity. McKibben — whose visionary leadership and tireless energy in this fight were awe-inspiring — provided the spark of inspiration that envisioned the tar sands as a galvanizing issue. Then the rest of the green movement jumped on board. While scrappier groups like 350.org and Rainforest Action Network helped organize direct action sit-ins, larger outfits like NRDC and Sierra Club focused on what they do best: reports, threats of litigation, and insider lobbying. Friends of the Earth deserves a special shout out for watch-dogging this issue long before it hit the headlines. It took the muscle of all of these groups — plus countless committed citizens — to pull off something as audacious as encircling the White House with more than 10,000 people.

The involvement of a number of unlikely bedfellows was also crucial. Ground zero for local opposition to the pipeline was that bastion of liberal sentiment, the state of Nebraska. Hats off especially to Jane Kleeb of Bold Nebraska, a grassroots progressive organization that organized ranchers, farmers, and the state’s conservative political leadership to demand re-routing the pipeline away from the cornhusker prairie.

Now compare that convergence of purpose — both within the environmental movement and beyond it — with the bickering and infighting that hamstrung the drive for comprehensive climate legislation. In a way, it’s an unfair comparison. Saying No is a lot easier than agreeing on the exact form of a 1,000-page piece of legislation. But that’s the point. No works, precisely because it’s firm and it’s strong and it’s uncompromising. No has the force of moral conviction. Think of other great moral efforts in US history — they almost always begin with a rejection. No to slavery. No to Jim Crow. No to the Vietnam War.

Some people say — with good reason — that environmentalism can’t just be about stopping things. It’s crucial to project a positive, hopeful, future-oriented message of what sustainability can look like. I totally agree. At the same time, I think that the Keystone XL victory shows that we also need much more NIMBY-ism. Think about it: If everyone said Not in My Backyard to giant oil pipelines, if everyone said Not in My Backyard to coal fired power plants or natural gas wells, then there would be no place to put them.

Rope a Dope the Opposition. If the Keystone XL victory shows how environmentalists can be more ambitious and energetic in their demands, it also demonstrates Republicans’ risk of overreach. When House Republicans attached a rider to the payroll tax extension calling on the president to make a decision on the pipeline within 60 days, many greens started wringing their hands and bemoaned that the vote was short-circuiting transparent decision making. A few folks, most notably Dave Roberts over at Grist, saw the flip side: the mandated speedup would most likely kill the project, as it wouldn’t give the State Department enough time to consider a re-route away from Nebraska.

That’s exactly what happened. Here’s White House spokesperson Jay Carney speaking to reporters on Tuesday: “It’s a fallacy to suggest that the president should sign into law something when there isn’t even an alternate route identified in Nebraska and when the review process is” not yet done. “There was an attempt to short-circuit the review process in a way that does not allow the kind of careful consideration of all the competing criteria here that needs to be done.”

Republicans have already telegraphed their intention to make EPA regulations a centerpiece of their 2012 campaign. I say, Bring It On. Poll after poll shows that Americans support laws to guarantee clean air and clean water. A survey last September found that more than two-thirds of Americans disagreed with Congress’ anti-EPA agenda. Greens can rope-a-dope the fossil fuel defenders once again by making defense of the EPA another line-in-the-sand test for candidates this year.

No Battle Is Ever Final. When the White House announced in November that it was postponing the pipeline decision to 2013, veteran environmental campaigner Glenn Hurowitz cautioned that the decision wasn’t so great. Either Obama wins re-election, Hurowitz wrote, and has a green light to approve the pipeline since he wouldn’t need environmentalists’ votes any longer; or Romney wins, and the pipeline goes through. I gave Hurowitz a little bit of grief (via snark machine Twitter) for spoiling the celebration, but he was right: Few victories are ever final.

Ditto today’s announcement. I just got off the phone with staffers with Friends of the Earth, and they cautioned that, as always, there will be devils in the details. TransCanada can always re-apply to build the pipeline with an expected re-route away from Nebraska’s Ogalla aquifer. A key question will be whether it has to do another full environmental impact report under NEPA (the National Environmental Protection Act). If it doesn’t have to do a new review, we can expect Keystone Battle Part II sometime in 2013. If it does have to complete an entirely new review, that would push the process a couple of years further into the future, likely killing it.

So stay vigilant, keep fighting, and remember that with today’s rejection we’ve the wind behind our backs.