In December 2019, the Danish government finished building a fence on the border with Germany. Although the fence was nominally intended to prevent wild boar entering Denmark, the government had recently acquired enough barbed wire to reinforce it against human migrants. I tell the story of the wild boar fence in the context of a global trend for escalating border enforcement and environmental change. I explore how border fences shape human and wild animal worlds, drawing on ecological data and using theory from environmental history, border and animal studies. In order to understand how humans and wild animals interact with the German-Danish border fence, I journeyed along it on foot in August 2022. My methodology is autobiographical – by walking the route myself and interviewing local experts and activists in the field, I explored how far humans and wild animals are free to move on the German-Danish border and what habitat fragmentation means for them. Without the ability to move, species worldwide, including humans, could be trapped as regions become uninhabitable due to climate change. A barbed wire border fence on the German-Danish border could prevent people, deer, wolves and other species from adapting to dramatic sea level rise and flooding. I argue that migration is the adaptation, rather than the crisis, and that mobility is something to be protected rather than supressed.