Sentence:  [T]hree ex-Secretaries of State for War had learnt their military theory under the <WSD>ægis</WSD> of Regular soldiers. Now it is an admitted fact that, broad-minded and enterprising as soldiers have frequently proved themselves in matters unconnected with the actual corporate body to which they belong, they are, nevertheless, perhaps the most obstinate and optimistic advocates of a "laissez-faire" policy, which the interests of their own profession are at issue, that it is at all possible to conceive. 
Meaning of the word marked with <WSD> tags:  "Usually as" '''under the aegis''': guidance, protection; endorsement, sponsorship.