The soldier is standing tall as he observes the passing people.
a soldier watching people looking for threats
Observing people doesn't imply he is looking for threats.
exists e2 e1 p1 s1 x1 x2.(n_people(x2) & Actor(e2,x2) & v_pass(e2) & n_male(x1) & n_soldier(x1) & exists e3.(Theme(e3,x2) & Actor(e3,x1) & v_observe(e3)) & as(e1,p1) & a_tall(s1) & Manner(e1,s1) & Actor(e1,x1) & v_stand(e1))
exists e2 e1 x1 x2 x3 x4.(a_topic(x1) & for(e1,e2) & for(e2,x4) & n_threat(x4) & Actor(e2,x2) & v_look(e2) & Theme(e1,x3) & Actor(e1,x2) & v_watch(e1) & n_people(x3) & (x1 = x2) & n_soldier(x1))