DALLAS — It was the afternoon before Earth Day in April when an imposing Republican stood up and declared war.

John Walsh III had spent the past half-hour sitting in the front row listening to former Democratic presidential candidate Wesley Clark, who happens to be a retired four-star general, try to convince the crowd that climate change is a national-security issue.

Then Walsh took the microphone.

"This is a war, and we need to treat it like one," he said. “I'm on the other side of the aisle from you politically, but I'm right in the trench with you on this issue.”

It was already a day of contrasts. A conservative had organized this Earth Day celebration. It attracted 100,000 people to Texas’ state fairgrounds, including climate researchers from elite universities as far away as New York City, oil-company executives, and families.