In different historical periods , the impacts of the five factors acting on the us national missile defense policy are different and involve the following four aspects : the influence degree which the five factors hold over us definite policies is different in a certain period ; in different era , the influence degree of a certain factor is also different on definite policies ; each factor may sometimes boost , but at the other time , constrain us national missile defense policy ; each factor may influence us process of developing national missile defense according to relatively objective existences or subjective judgement 这些影响具体表现在如下4个方面：在同一时期表现为不同的因素对最终政策的影响程度不同；每个因素既可能表现为推动作用，也可能表现为制约作用；每个因素既可能从相对的客观存在出发，也可能从主观的判断出发影响美国发展国家导弹防御的进程；在不同的时期表现为，同一因素对最终的政策影响程度可能不同。
