The Nazgûl riding on hideous winged creatures
From the outset of WW2 Britain was the target of the Blitz - mass air attacks against industrial targets, towns and cities by the Third Reich's air force, the Luftwaffe.
Tolkien himself witnessed the bombing of Coventry (a city fifty miles north of Oxford). He was a part-time air-raid warden and member of the Firewatching Service1 and while I won't go that far as to compare the winged Nazgûl to the Luftwaffe and the Eagles to the British RAF, (in which his son, Michael fought as an anti-aircraft gunner)
It is very probable that Tolkien was influenced by the terror of the night air raids and required blackouts.
Probably that's why we see Gondor surrounded by darkness and targeted from the air by evil beings, the Ringwraiths, which now acquired wings:
...winged terror, the Nazgûl. He shuddered, and hope seemed to wither.
And even at that moment the sun for a second faltered and was
obscured, as though a dark wing had passed across it. Almost beyond
hearing he thought he caught, high and far up in the heavens, a cry:
faint, but heart-quelling, cruel and cold. He blanched and cowered
against the wall. 'What was that?' asked Beregond. 'You also felt
something?' 'Yes,' muttered Pippin. 'It is the sign of our fall, and
the shadow of doom, a Fell Rider of the air.' 'Yes, the shadow of
doom,' said Beregond. 'I fear that Minas Tirith shall fall. Night
comes. The very warmth of my blood seems stolen away.'
~The Return of the King: Minas Tirith
After all, air raids are scary. Hearing the alarm for the first time, especially if you caught out of shelter in the dark can be an unforgettable experience. That's why I think they were a contributing factor of Tolkien describing so masterfully and emphatically the nazgul attacks.
German two-engined Heinkel III bombers fly to attack - Battle of Britain, 1940
no matter which side you are on, air raids are terrifying:
THE ENEMY SEES YOUR LIGHT! GO DARK! - German air raid warning poster about 1943
On the influence of WW2 in general:
Tolkien scholar Tom Shippey describes The Lord of the Rings as:
a war-book, also a post-war book, framed by and responding to the
crisis of Western civilization, 1914–1945 (and beyond)**~The Hobbit Party: The Vision of Freedom That Tolkien Got, and the West Forgot, Jay Richards, Jonathan Witt
In the article The Complexity of Tolkien's Attitude Towards the Second World War various experts cited expressing similar opinions, among them
Edmund Fuller, novelist, historian and literary critic:
It has for me an allegorical relation to the struggle of Western
Christendom against forces embodied, successively but overlapping, in
Nazism and Communism. The work was conceived and carried forward when
the darkest shadow of modern history was cast over the West and, for a
crucial part of the time, over England in particular.
Tolkien himself denied that the LoTR made any reference to the Second World War. In the preface to LOTR’S Second Edition he presents two arguments :
The crucial chapter, "The Shadow of the Past', is one of the oldest
parts of the tale. It was written long before the foreshadow of 1939
had yet become a threat of inevitable disaster, and from that point
the story would have developed along essentially the same lines, if
that disaster had been averted. Its sources are things long before in
mind, or in some cases already written, and little or nothing in the
war that began in 1939 or its sequels modified it.
The real war does not resemble the legendary war in its process or its
conclusion. If it had inspired or directed the development of the
legend, then certainly the Ring would have been seized and used
against Sauron; he would not have been annihilated but enslaved, and
Barad-dûr would not have been destroyed but occupied."~The Lord of the
Rings - Foreword to the Second Edition
Let's start with the second argument. This is the weaker of the two, and when I read it, I wondered why JRRT even bothered to make it. Tolkien essentially points out that LoTR is not an one-to-one recount of the events of WW2- the Allies would have used any tool or technology available to win the war, and their victory was followed by an occupation of the defeated enemy. But the thing is, the existence of allegorical or satirical novels or animal fables which are intentional one-to-one account of the events they portray doesn’t prove the opposite: That is, if a novel is not a completely faithful recount of some evens, then it is not inspired by those events. After all, artistic freedom is a thing, and the writer is luckily not bound by international treaties and alliances so he can write whatever solution to the conflict he pleases for his own story. Obviously “the ring made me do it” is not a valid excuse in the real world but
in a secondary universe the author can toss the externalized source of all problems in the form of jewelry into a fiery volcano so everyone could live happily and without temptation.
Unlike novels such as Orwell's Animal Farm which was an allegory and satire of the events leading up to the Russian Revolution of 1917 and then on into the Stalinist era of the Soviet Union2, JRRT's work is in no way a conscious effort to write such a novel about WW2. That doesn’t mean we can’t look for parallels, yet Tolkien seems to discourage the reader from doing so, by essentially stating that the chapter which set up the trilogy was written ‘long before 1939’ that is, before the start of WW2. This argument would have been very convincing, if up until 1939 the nations had lived together in harmony with no warning signs whatsoever, and the Fire Nation Third Reich’s attack on Poland and the subsequent entry of the Britain (and France) into the war was a big surprise for everyone in Britain, and not, say, a result of a continuously escalating conflict featured in the headlines of newspapers worldwide since the rise of the Nazi party to power (1932-3), which soon followed by Hitler leaving the Disarmament Conference and the League of Nations (1933) Hitler rearming Germany (1933-1935) Hitler supporting Mussolini in the Abyssinian crisis (1935) Hitler sending troops to Rhineland (1936) Hitler annexing Sudetenland then Czechoslovakia and Austria (1938) to mention just a few events.
Nowadays we calm ourselves that nuclear proliferation and the powers’ capability of second strike makes the threat of a global war less likely, as ‘there can be no winners in a nuclear war’ but in the mid-to late 1930s that was not the case and the citizens of Britain were well aware of the threatening possibility of another great war breaking out fought with conventional weapons, and were scared shitless understandably concerned by this perspective, and regardless if they put their faith in the success of Chamberlain’s Appeasement Policy or were convinced that said policy was a trainwreck waiting to happen, people generally were aware that the political situation was extremely fragile.
Let’s look at some political cartoons of the 1930s of the then very famous cartoonist David Low, who was so popular with the general public that media magnate Lord Beaverbrook, employed him to work at the Evening Standard. Low produced four cartoons a week and these were syndicated to 170 journals worldwide.
Stepping Stones to Glory, 1936

The cartoon features Hitler marching towards his goal (‘Boss of the Universe’) along a carpet laid across the backs of the ‘spineless leaders of democracy’. The cartoon was a response to Hitler sending troops into the Rhineland, a demilitarised zone under the Treaty of Versailles. Britain at that time was pursuing her policy of appeasement, so there were no repercussions.
Peace Gesture, 1936

Nazi Tourist Line

You Have Been Warned, 1938

As early as in 1933–4 most British MPs were aware that Germany was secretly rearming and thus becoming an increasing threat. In March 1934 Churchill said:
Germany is arming fast and no one is going to stop her. That seems
quite clear. No one proposes a preventive war to stop Germany breaking
the Treaty of Versailles. She is going to arm; she is doing it; she
has been doing it. I have no knowledge of the details, but it is well
known that those very gifted people with their science and with their
factories … are capable of developing with great rapidity the most
powerful Air Force for all purposes, offensive and defensive, within a
very short period of time. I dread the day when the means of
threatening the heart of the British Empire should pass into the hands
of the present rulers of Germany. *~The Gathering Storm, Winston Churchill
Sir John Simon, the Foreign Secretary, admitted that:
German civil aviation is now the first in Europe. Germany already has
in effect a fleet of 600 military aeroplanes and facilities for its
very rapid expansion. She can already mobilise an army three times as
great as that authorised by the Treaty.
and after a few tumultuous years despite all the diplomatic efforts by 1939 it was crystal clear that the League of Nations, (the predecessor to the United Nations) which was created to prevent devastating wars like WW1, completely failed in its purpose to act as an arbitrator in settling conflicts between nations, Britain was building up his air force and strengthening its navy and was desperate enough to seek an alliance with the hated commies Soviet Union.
The interwar period, and especially the 1930s were a very troubling time with impassioned and vociferous public debate and criticism of Chamberlain’s appeasement policy. People lived under the shadow of the threat Nazi Germany presented, and it was in this political climate that Tolkien set out to write a sequel to the well received Hobbit, which was published in 1937:
Since The Hobbit was a success, a sequel was called for; and the
remote Elvish Legends were turned down. A publisher's reader said they
were too full of the kind of Celtic beauty that maddened Anglo-Saxons
in a large dose. Very likely quite right. ~Letter 163 to W.H. Auden(1955)
It is curious, that in the chapter Shadows of the Past, the earliest chapter which sets up the story, Frodo laments that they live in probing times:
“But last night I told you of Sauron the Great, the Dark Lord. The
rumours that you have heard are true; he has indeed arisen again and
left his hold in Mirkwood and returned to his ancient fastness in the
Dark Tower of Mordor. That name even you hobbits have heard of, like a
shadow on the borders of old stories. Always after a defeat and a
respite, the Shadow takes another shape and grows again.” “I wish it
need not have happened in my time,” said Frodo. “So do I,” said
Gandalf, “and so do all who live to see such times.~The Fellowship of the Ring- Chapter 2: The Shadow of the Past (emphasis mine)
and
But that is not for them to decide. All we have to decide is what to
do with the time that is given, us. And already, Frodo, our time is
beginning to look black. The Enemy is fast becoming very strong. His
plans are far from ripe, I think, but they are ripening. We shall be
hard put to it. We should be very hard put to it, even if it were not
for this dreadful chance.~ibid(emphasis mine)
Therefore it is not surprising that according to the opinion of various Tolkien scholars and literary critics JRRT was strongly affected (and how could he not) by his traumatic WW1 experiences, by the public debate on the political situation in the interwar period and by WW2 itself while planning and writing the LoTR triology. Many critics attribute the widespread success of the trilogy to its portrayal of a fight of good versus evil, the just war, not unlike how WW2 was portrayed by the British newspapers.
While I prefer more the Death of the Author approach, I should note that different ones are also perfectly valid, such as the readers asking questions on what inspired the author, submerging themselves into the era the author lived white creating the work, reading correspondence with contemporaries, digging up newspapers, caricatures and propaganda posters, reading books about the period. It is also a worthwhile experience.
1. Colin Duriez- J.R.R. Tolkien: The Making of a Legend - Tolkien's Second War
2. Animal Farm - Wikipedia