Assalamu 'Alaikum wa rahamtullahi wa barakatuhu
I found this in the Greatest ever Polemical site to roam the face of this Cyberspace, otherwise known as www.islamic-awareness.org
history narrates to us the famous attempt of a great Christian army to conquer Mecca. This army from Yemen was supported by elephants and was lead by Abraha, the Abyssinian. The doomed invasion occurred in the same year that the Prophet Muhammad(P) was born ,and later came to be known amongst the Arabs as the Year of the Elephant. The army's aim was to vanquish Mecca, destroy the Ka'aba (the holy shrine built by Abraham(P) and his son Ismācīl(P)) and then to convert the pagan Arabs to Christianity. Once this was accomplished, they could force them to make pilgrimage to the great church named al-Qulais that Abraha had built in Yemen for this purpose.
Ibn Ishāq in al-Sīrah al-Nabawiyyah under the title The Story of the Elephant said:
Then Abraha built the "Qulais" in San'a, it was a church that people never saw its like in their time, then he wrote to the Abyssinian king; "I built for you O king, a church that no king had before you, and I'll not stop until I make the Hajj - that the Arabs perform to Ka'aba - shifted to it..."
Dr Helmi Mahroos Ismācīl in his book al-Sahrq al-'Arabi al-Qadīm writes:
Abraha worked hard on spreading Christianity among the Yemens, he built many churches there the most important of it all was the "Qulais" in Sana'a which the Abyssinian took as their capital in Yemen. Abraha tried to make the Arabs to perform Hajj to it.
This Christian attempt lead by Abraha to destroy the Ka'bah corroborates that Mecca and al-Hijaz, in general, had no Christian or Jewish influence whatsoever even until the time the Prophet Muhammad(P) was born. Abraha had failed in his attempt to destroy the Ka'bah and this was a subject of a Qur'ānic Chapter as a Sign from God (Surat al-Fīl) :
Seest thou not how thy Lord dealt with the Companions of the Elephant? Did He not make their treacherous plan go astray? And He sent against them flights of Birds Striking them with stones of baked clay. Then did He make them like an empty field of stalks and straw (of which the corn) has been eaten up. [Qur'ān: 105]
Historians could not explain how this great army of Abraha didn't reach its goals in conquering the weak- and almost surrendered city of Mecca!!
The following provides an excerpt from a Yemenite archaeological site that mentions, in part, this incident. Walter W. Muller, a specialized researcher in ancient Arabian history, under the subject Outline of the History of Ancient Southern Arabia, says:
Muller says: "Southern Arabia became an Abyssinian dominion, first under the local Christian vassal simyafa then under the former Abyssinian General Abreha (Abraha). In 542 .... An inscription dated 547, reporting of a campaign against the rebellious Maadd in Central Arabia (Ry 506). States that Abreha had already styled himself king. The most recently dated inscription of the Himyarite era (CIH 325) is from A.D. 554. It virtually marks the end of the well-documented ancient Southern Arabian epoch and heralds the decline of the Sabeo-Himyarite empire..... Towards the end of his reign, Abreha launched yet another military campaign against the North which has been preserved in the memory of the Arabs because of the elephants accompanying it. Abreha failed to take Mecca as he had intended and the operation had to be abandoned."
Without giving any reason why Abraha had failed in capturing Mecca even though it had surrendered!
Bernard Lewis in his book The Middle East: 2000 Years Of History From The Rise Of Christianity To The Present Day, writes:
Newly converted, the Ethiopians were fervent in their Christianity and responded eagerly to Byzantine embassies. Unfortunately for the Ethiopians, they were not able to complete the task assigned to them. They succeeded initially in crushing and destroying the last independent state in southern Arabia, and opening the country to Christian and other external influences, but they were not strong enough to maintain it. They had even tried to advance northwards from the Yemen, and in 507 CE had attacked Mecca, a Yemenite trading post on the caravan route to the north. The Ethiopians failed and were defeated, and a little later the Persians came to the Yemen in their place.
For further information on the story of this Christian campaign against Mecca a reference is made in al-Seerah al-Nabawiyyah. Below is a pre-Islamic poem from the same source that preserved the event by a person who had witnessed it. The poet Nufail bin Habeeb was there when the event took place and met with the fleeing soldiers of Abraha's army who asked him the directions to Yemen:
The poem can be roughly translated as follows:
Greetings Rudainah (A female name) we have been pleased with an early morning view. We had received - a seeker of fire - from your side (the word Qabis is used for a person who seeks fire or wood to be used as a source of light at night) but he could not find anything here. O Rudainah! if you have seen what we have seen near al-Muhasab (a location between Mecca and Mina) you would excuse me and not be saddened with what happened in the past between us. I thanked God when I saw the birds and I was afraid of stones that was thrown on us. And they (Abraha's men) were asking about me (to show them the way) as if I was owing them some previous debts.
 Ibn Hishām, al-Sīrah al-Nabawiyyah, Mousasat cUlūm al-Qur'ān, Beirut,, p.43.
 Helmi Mahroos Ismael, al-Sahrq al-cArabī al-Qadīm, 1997, Mousasat Shabab al-Jami'ah, Egypt, p. 210 - 211.
 Bernard Lewis, The Middle East: 2000 Years Of History From The Rise Of Christianity To The Present Day, 1996 (Second Impression), Phoenix: London, p.45
 Ibn Hishām, Op.Cit, p.53.
So to sum up:
1. Historians and Researchers agree that Abraha led a campaign against Makkah., in an evil bid to destroy the Kaa'ba.
2. His plan however was foiled. Not only so, they were in fact defeated, and persians took their position.
3. The question that needs to be asked now: how come the Great Abraha, Leader of Yemen, with all its might, and that of the Elephants which the Makkans have never seen before, could not conquer the tiny city of Makkah? It was a mere tiny city, having tribes squabbling amongst themselves, against the Great Might the like of which Makka has never before seen. Can there be any naturalistic explantion for this? I for one, doubt.
4.To top it off, there are pre-Islamic poetry which talk about this event and they mention the birds and the stones in specific.
5. Combining the two pieces of Evidence, we could say that it is very plausible that the Event narrated of Surah Fil has sound historical basis.
Allah knows best.