9 5 0 s ▁and ▁ 6 0 s , ▁the ▁US ▁Army ▁had ▁been ▁developing ▁AB M ▁systems ▁as ▁a ▁defense ▁against ▁the ▁IC B M . ▁At ▁first ▁these ▁were ▁task ed ▁with ▁counter ing ▁a ▁force ▁of ▁do z ens ▁or ▁maybe ▁ 1 0 0 ▁war head s , ▁target ed ▁against ▁US ▁cities ▁and ▁large ▁industrial ▁sites . ▁This ▁was ▁seen ▁as ▁a ▁relatively ▁simple ▁task ▁not ▁unlike ▁the ▁Army ' s ▁existing ▁anti - air craft ▁role . ▁However , ▁as ▁the ▁numbers ▁of ▁IC B M s ▁in ▁the ▁Soviet ▁forces ▁rapidly ▁grew , ▁the ▁price ▁of ▁implementing ▁such ▁a ▁system ▁quickly ▁ran ▁out ▁of ▁control , ▁reaching ▁$ 4 0 ▁billion ▁($ ▁billion ▁in ▁), ▁a ▁significant ▁amount ▁of ▁the ▁total ▁budget ▁of ▁the ▁US ▁Department ▁of ▁Def ense . ▁ ▁AR PA ▁was ▁asked ▁to ▁consider ▁the ▁problem ▁of ▁using ▁the ▁technology ▁in ▁a ▁cost - effect ive ▁way , ▁and ▁noticed ▁one ▁possible ▁application . ▁As ▁Soviet ▁war head s ▁were ▁accurate ▁to ▁within ▁only ▁a ▁few ▁kilom eters , ▁and ▁they ▁had ▁to ▁fall ▁within ▁less ▁than ▁a ▁kilometer ▁to ▁kill ▁a ▁miss ile ▁sil o , ▁that ▁meant ▁the ▁S ovi ets ▁would ▁have ▁to ▁launch ▁several ▁war head s ▁at ▁every ▁US ▁sil o ▁in ▁a ▁counter force ▁attack . ▁The ▁def ender ▁had ▁a ▁significant ▁advantage ▁here ; ▁they ▁could ▁watch ▁the ▁incoming ▁war head s , ▁see ▁if ▁any ▁were ▁going ▁to ▁come ▁within ▁the ▁le th al