close ranks

Short definition from Wiktionary

Verb

1. 1
   To close the spacing between lines (ranks) of troops, as they are deployed across a battlefield, thereby decreasing the depth of the formation as it faces the enemy side.
2. 2
   To regroup forces, especially when this involves overlooking differences in order to face a challenge or adverse situation. Often implies making a show of unity, especially to the public.
3. 3
   To maintain silence and secrecy in the face of some external threat.