【Maintain】 , 【assert】 ,  【defend】 ,  【vindicate】 ,  【justify】  are comparable when they mean to uphold as true, right, just, valid, or worthy of notice or acceptance in the face of opposition or indifference.

【Maintain】  implies a firmness of conviction. When this implication is the only one,  【maintain】 usually means to argue in the spirit of one who does not admit any weakness in his contention. Often, however, the term additionally implies persistency or insistency in upholding in defiance of all opposition.

【Assert】  (see also  【ASSERT】 1 ) so strongly implies a determination to make others accept or recognize what one puts forward as the truth, or as a claim, or as a right, that it often suggests aggressiveness or obtrusiveness. But  【assert】 does not always imply the use of argument to force conviction or recognition.

【Defend】  implies a maintaining in the face of attack with the intention of demonstrating the truth, rightness, or propriety of what is questioned; thus, one defends a thesis who, as a candidate for a degree, submits himself to examiners who assail the weak or dubious points of his argument.

【Defend】 , in this sense, does not imply, as it so often implies in its more common sense (see  【DEFEND】 1 ), that the defender is in a weak or dubious position; however it seldom suggests as much aggressiveness as does  【assert】 and often connotes the aim of an apologist.

【Vindicate】  (see also EXCULPATE ) implies an attempt, usually a successful attempt, at defense or assertion. It presupposes that whatever is being defended or asserted has been or is capable of being challenged, questioned, denied, or contemned. When the emphasis is on defense, then argument or something which has the force of argument is usually implied, and an aim not only to make one’s point but to confute and confound one’s opponents is often connoted.

When the emphasis is upon assertion,  【vindicate】 usually implies an effort to resist triumphantly the force of encroachment or interference or to overwhelm those who deny or doubt, not so much by argument as by appropriate action.

【Justify】  (see also EXPLAIN 2  【JUSTIFY】 3 ) implies that the thing concerned can no longer be opposed or ignored because it has been conclusively shown to be true, valid, or proper by irrefutable arguments or on inescapable grounds, such as its consequences or its successful operation.