His primary concern was ensuring that the pilots had a clear understanding of their rules of engagement.
( ( His ( primary concern ) ) ( ( was ( ensuring ( that ( ( the pilots ) ( had ( ( a ( clear understanding ) ) ( of ( ( their rules ) ( of engagement ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP$ His) (JJ primary) (NN concern)) (VP (VBD was) (VP (VBG ensuring) (SBAR (IN that) (S (NP (DT the) (NNS pilots)) (VP (VBD had) (NP (NP (DT a) (JJ clear) (NN understanding)) (PP (IN of) (NP (NP (PRP$ their) (NNS rules)) (PP (IN of) (NP (NN engagement))))))))))) (. .)))
The pilots had previously been briefed on the rules of engagement.
( ( The pilots ) ( ( ( had previously ) ( been ( briefed ( on ( ( the rules ) ( of engagement ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NNS pilots)) (VP (VBD had) (ADVP (RB previously)) (VP (VBN been) (VP (VBN briefed) (PP (IN on) (NP (NP (DT the) (NNS rules)) (PP (IN of) (NP (NN engagement)))))))) (. .)))