How to loop over this dictionary in Ansible?

Hows this

- hosts: localhost
  vars:
    war_files:
      server1:
      - file1.war
      - file2.war
      server2:
      - file1.war
      - file2.war
      - file3.war
  tasks:
    - name: Loop over subelements of the dictionary
      debug:
        msg: "Key={{ item.0.key }} value={{ item.1 }}"
      loop: "{{ war_files | dict2items | subelements('value') }}"

dict2items, subelements filters are coming in Ansible 2.6.

FYI, if a filter for your objective doesn't exist, you can write your own in python without having to resort to jinja2 hacks. Ansible is easily extendable; filters in filter_plugins/*.py are searched by default adjacent to your plays/roles and are automatically included - see Developing Plugins for details.

Now Ansible allows this

- name: add several users
  user:
    name: "{{ item.name }}"
    state: present
    groups: "{{ item.groups }}"
  with_items:
    - { name: 'testuser1', groups: 'wheel' }
    - { name: 'testuser2', groups: 'root' }