▁February ▁and ▁April ▁ 1 9 7 1 , ▁the ▁US ▁and ▁South ▁Vietnam ese ▁Army ▁of ▁the ▁Republic ▁of ▁Vietnam ▁( AR V N ) ▁carried ▁out ▁Operation ▁Lam ▁Son ▁ 7 1 9 , ▁the ▁invasion ▁of ▁La os ▁in ▁an ▁effort ▁to ▁dis rupt ▁the ▁Ho ▁Chi ▁Min h ▁Tra il . ▁Over ▁a ▁period ▁of ▁time , ▁the ▁North ' s ▁People ' s ▁Army ▁of ▁Vietnam ▁( PA V N ) ▁was ▁able ▁to ▁am ass ▁a ▁significant ▁counter - in vas ion ▁force , ▁including ▁extremely ▁strong ▁anti - air craft ▁forces ▁and ▁mass ed ▁art illery . ▁As ▁these ▁forces ▁began ▁to ▁push ▁AR V N ▁out ▁of ▁La os , ▁Fire ▁Base ▁ 3 1 ▁was ▁the ▁subject ▁of ▁a ▁classic ▁arm or ▁attack ▁by ▁T - 3 4 , ▁T - 5 5 ▁and ▁P T - 7 6 ▁t anks . ▁ ▁This ▁attack ▁was ▁met ▁by ▁ro cket - f iring ▁Cob ras . ▁One ▁post - action ▁report ▁noted ▁that ▁ 4 7 ▁t anks ▁were ▁attacked , ▁and ▁every ▁one ▁respond ed ▁to ▁the ▁hel ic opter ▁with ▁its ▁ 1 2 . 7   mm ▁machine ▁gun , ▁but ▁not ▁one ▁hel ic opter ▁or ▁crew man ▁was ▁lost . ▁The ▁hel ic op ters ▁out right ▁destroyed ▁six ▁t anks , ▁had ▁nin ete en ▁" m ob ility ▁k ills " ▁and ▁a ▁further ▁eight ▁dam aged . ▁See ing ▁this ▁outcome , ▁Army ▁command ers ▁noted