When you encapsulate the stage inside a node, everything will run on that node. Thus the war will ne destroyed with the container when the build finished.
You have several alternatives to keep the war.
The maven way is to deploy artifacts to a repisitory such as nexus.
Another jenkins alternative is to stash the war on the slave once the build finishes and un stash it back on the master. This will effectively copy back the stashed files from the slave to the master node.