There has been much interest over the centuries about the "prophecies" made by the 16th century almanac writer Nostradamus (born Michel de Notredame.) Popularly believed to be a doctor, he in fact failed and dropped out of the medical school of his day. He also refused to accept the title of "prophet" even during his life. This was mostly to save himself from persecution by the Inquisition and the Catholic Church, which held a great deal of both spiritual and secular power at the time. According to the pre-eminent Nostradamus expert, Peter Lemesurier, the most likely method Nostradamus used in formulating his oblique and rambling "quatrains" was bibliomancy and knowledge of history.
Bibliomancy was very popular in the 16th century. One found the ideas for their work by opening a book at random and using the first word or sentence to begin. And as any student of history knows, new political leaders will arise and be compared to past leaders and some of them will be assassinated. Predicting new wars is easy since warfare has always been a popular human pastime. What goes around comes around and just a quick calculation of where the astrological elements are when one event happens will give a distant future time when "everything repeats itself again."

A 16th-century edition of predictions by Nostradamus is displayed by a librarian in this January 15, 2010 file picture. Credit: Reuters/Robert Pratta/Files
Then there is the method of writing that Nostradamus used. It was a combination of archaic, even in his day, French, Spanish, Latin, and Hebrew, along with coded references, and mystic sounding sensationalism to appeal to the readers of his almanacs, most notably the French Royal Court. Then can be added the problems caused by typesetters that were sometimes careless in their setting and did not follow the copy given them.
An example from the most likely candidate for a prophecy of alien invasion, Verse 72 of the Tenth Century is the phrase "Du ciel" usually translated as "from the sky" when the same phrase is used frequently by Nostradamus to mean "from the region." In the same Verse, the corrected word "deffraieur," meaning "provider" was initially replaced by a typesetter with the word "d’effrayeur," which means "of terror." Finally, Nostradamus did not give the dates in his writings based on the traditional calendar years but dated them from the time of the first Council of Nicaea in 325 A.D. So the year 1999 in the Verse would actually figure out to 2324 A.D.
Translation can also add to the confusion of trying to determine what the supposed prophecy said. In its original form Verse 72, Century Ten says:
L'an mil neuf cens nonante neuf sept mois,
Du ciel viendra un grand Roy d'effrayeur,
Resusciter le grand Roy d'Angolmois,
Avant apres, Mars regner par bon heur.
In the book "The Complete Prophecies of Nostradamus" by Henry C. Roberts it is translated thusly:
In the year 1999 and seven months
From the skies shall come an alarmingly powerful king,
To raise again the great King of the Jacquerie,
Before and after, Mars shall reign at will.
Peter Lemesurier translated the Verse as:
When 1999 is seven months o'er
From thereabouts shall a great hosting King
Restore the King from Angoumois once more,
Who'll reign propitiously once come the spring.
And Google Translate gives us this version:
The year one thousand nine hundred ninety nine seven months
From the sky will come a great King of Terror,
Life the great King of Angolmois,
Before after Mars to reign by good luck.
There are numerous other translations and just as many varying interpretations. Many believe this prophecy denotes the return of the Annunaki and the subsequent restoration of their rule on Earth, complete with the restarting of the wars they were so frequently party to among themselves as chronicled in many of the ancient Sumerian cuneiform texts. It didn't seem to happen in 1999 A.D. although the faithful may look for it to occur in 2324 A.D.
People love puzzles and it is quite likely that much of Nostradamus' writings were for the entertainment of his readership as they sought their own interpretations of his oblique words. No one has been able to point to any of these predicted happenings before they occurred. It has always been through the process of looking for some clue in the Verses that correlates to a past event that any supposed meaning is determined. It is much like the process of pattern recognition the brain uses to see images in the clouds.

Did Nostradamus predict UFO's and extraterrestrial alien invaders? We will most likely have to wait until they make a public landing or re-establish their rule over us humans before someone can look through the prophecies and find the one that sounds most fitting for the event, then they can say with self-assurance that he did indeed predict it.