How many insurgents are there when the total per period is 156?
CREATE TABLE "table1_21636599_2" (
  "period" real,
  "civilians" text,
  "security_forces" text,
  "insurgents" text,
  "total_per_period" real
);
SELECT "insurgents" FROM "table1_21636599_2" WHERE "total_per_period"=156;