Were the attempts by southern states to restrict african-american voting legal under the fifteenth amendment?

yes, because the amendment did not provide legislation to enforce the universal right to vote.
yes, because the amendment allowed individual states to determine how they provided voting rights.
no, because the amendment guaranteed the ability to vote for all people.
no, because the amendment included legislation forbidding the restriction of voting rights.

I believe the answer is: Yes, because the amendment did not provide legislation to enforce the universal right to vote.

Since there is nothing to enforce the right, the southern states could create a legal 'requirement' to vote which would eliminate the majority of african American voters.

At that time, the pass the legislation that require literacy test to be eligible for voting. This made almost all African American unable to vote since they did not obtain any access to education during slavery.

to force terrorists groups to stop attacking western targets using massive military force.