You don't know the difference between a transitive verb and a noun, do you?

The Merriam-Webster definition provides more detail in separate entries for the verb and noun forms of "retard":

Main Entry: re·tard
Pronunciation: ri-'tärd
Function: verb
Etymology: Middle English, from Middle French or Latin; Middle French retarder, from Latin retardare, from re- + tardus slow
Date: 15th century
transitive senses
1 : to slow up especially by preventing or hindering advance or accomplishment : IMPEDE
2 : to delay academic progress by failure to promote
intransitive senses : to undergo retardation

If you substitute the verb-transitive synonym "impede" in your example, you might finally get it:

The anti-war protesters were attempting to impede the war effort.

Note that there's no intranstive-verb form, so your next example is gramatically incorrect:

The anti-war protesters were retarding.

There needs to be an object which they are retarding; the action can't dangle there in thin air!

You were laboring mightily to insult me with the noun sense:

Main Entry: re·tard
Function: noun
Date: 1788
1 /ri-'tärd/ : a holding back or slowing down : RETARDATION
2 /'rE-"tärd/ often offensive : a retarded person; also : a person held to resemble a retarded person in behavior

Didn't work very well, did it? There's nothing lamer than having to fix somebody else's insult. Your belly must be beet-red about now!

--Robert