Gather round folks, 'cuz today is geography day.
This cover was posted in Willemstad on the island of Curaçao in the Netherlands Antilles.

What's a Netherlands Antilles, you ask?
The Netherlands Antilles was formerly the Dutch colony Curaçao off the coast of Venezuela.
There were three principal islands; Aruba, Curaçao, and Bonaire.
There are also a couple of tiny islands, Saba and Saint Eustatius,  squeezed in adjacent
to Saint Christopher-Nevis south of Anguilla in the northerly Leeward islands.

Saint Eustatius has a unique place in American history.
It was the place where an American naval vessel received the first naval salute recognizing American independence.
The Dutch were at odds with the British at the time so it suited their purposes to support the Americans.
The details of this relationship and the event are well told in Barbara Tuchman's book, The First Salute.

The Netherlands Antilles were made an integral part
of the Kingdom of the Netherlands under the 1954 constitution.
In 1986, Aruba achieved separate status and is no longer part of the Netherlands Antilles.

The cover received a machine cancel with an illegible slogan to the left.
The cover was imprinted with a red IMPRIMÉ.
This indicated that the cover qualified for a printed matter rate.
The cover carried some sort of advertising or institutional mail.

It appears that its destination in Nigeria brought an immediate response from the postal clerks.
The cover was given a second handstamp the same day, and marked
with two double straight line handstamps SERVICE SUSPENDED | RETURN TO SENDER.
I cannot explain why this marking is in English.
Maybe someone else can.

There is another explanation for this marking.
The second Willemstad handstamp may have been used to cancel the left-most
stamp which appears to have been missed by the machine cancel.
If the cover actually got to Nigeria, the straight line handstamps would
have been in English which is the official language there.*
I don't know what the UPU regulations are regarding undeliverable printed matter however.
I would think that they do not require it to be returned to sender.*

The destination was to have been Owerri.
Owerri is on the east side of the Niger River delta about 140 km
from the Bight of Biafra at the east end of the Gulf of Guinea on the Atlantic Coast of Africa.
Owerri is also about 140 km from Onitsha which is at the head of the Niger River delta.

The reason that the service was suspended was the Biafran War which was going on at the time.

The cover is franked with three stamps from the 1965 flag series:
3c Lace, Saba in chalky blue, ultramarine and red (Scott 297),
2c Divi-Divi tree and Haystack Mountain, Aruba, in yellow, ultramrine and red (Scott 296),
and 5c Church ruins, St. Eustatius, in light blue, ultramarine and red (Scott 299).

*Thanks to David Frick for suggesting this alternate line of thinking.