Question: When did construction begin on the Great Wall of China?
About the article "Great Wall of China", section "Abstract".

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In China, one of the first individuals to attempt a multi-dynastic history of the Great Wall was the 17th century scholar Gu Yanwu.
More recently, in the 1930s and 1940s, Wang Guoliang (王國良) and Shou Pengfei (壽鵬飛) produced exhaustive studies that culled extant literary records to date and mapped the courses of early border walls.
However, ultimately these efforts were based solely on written records that contain obscure place names and elusive literary references.
The rise of modern archeology has contributed much to the study of the Great Wall, either in corroborating existing research or refuting it.
However these efforts do not yet give a full picture of the Great Wall's history, as many wall sites dating to the Period of Disunity (220589) had been overlaid by the extant Ming Great Wall.
Western scholarship of the Great Wall has, until recently, been affected by misconceptions derived from traditional accounts of the Wall.
When the Jesuits brought back the first reports of the Wall to the West, European scholars were puzzled that Marco Polo did not mention the presumably perennial "Great Wall" in his "Travels of Marco Polo".
Some 17th century scholars reasoned that the Wall must have been built in the Ming Dynasty, after Marco Polo's departure.
However, this view was soon overtaken by another that argued, against Polo's own account, that the Venetian merchant had come to China from the south and so did not come into contact with the Wall.
Thus, Father Martino Martini's mistaken claim that the Wall had "lasted right up to the present time without injury or destruction" since the time of Qin was accepted as fact by the 18th century "philosophes".
Since then, many scholars have operated under the belief that the Great Wall continually defended China's border against the steppe nomads for two thousand years.
For example, the 18th century sinologist Joseph de Guignes assigned macrohistorical importance to such walls when he advanced the theory that the Qin construction forced the Xiongnu to migrate west to Europe and, becoming known as the Huns, ultimately contributed to the decline of the Roman Empire.
Some have attempted to make general statements regarding Chinese society and foreign policy based on the conception of a perennial Great Wall: Karl Marx had the Wall represent the stagnation of the Chinese society and economy, Owen Lattimore supposed that the Great Wall demonstrated a need to divide the nomadic way of life from the agricultural communities of China, and John K. Fairbank posited that the Wall played a part in upholding the Sinocentric world order.
Despite the significance that the Great Wall seemed to have, scholarly treatment of the Wall itself remained scant during the 20th century.
Joseph Needham bemoaned the dearth when he was compiling the section on walls for his "Science and Civilization in China": "There is no lack of travelers' description of the Great Wall, but studies based on modern scholarship are few and far between, whether in Chinese or Western languages."
In 1990, Arthur Waldron published the influential "The Great Wall: From History to Myth", where he challenged the notion of a unitary Great Wall maintained since antiquity, dismissing it as a modern myth.
Waldron's approach prompted a re-examination of the Wall in Western scholarship.
Still, as of 2008, there is not yet a full authoritative text in any language that is devoted to the Great Wall.
The reason for this, according to "The New Yorker" journalist Peter Hessler, is because the Great Wall fits into neither the study of political institutions (favoured by Chinese historians) nor the excavation of tombs (as favoured by Chinese archeologists).
However, the void left by academia is being filled by independent research from Great Wall enthusiasts such as ex-Xinhua reporter Cheng Dalin (成大林) and self-funded scholar David Spindler. 
The Xinhai Revolution in 1911 forced the abdication of the last Qing Emperor Puyi and ended China's last imperial dynasty.
The revolutionaries, headed by Sun Yat-sen, were concerned with creating a modern sense of national identity in the chaotic post-imperial era.
In contrast to Chinese academics such as Liang Qichao, who tried to counter the West's fantastic version of the Great Wall, Sun Yat-sen held the view that Qin Shi Huang's wall preserved the Chinese race, and without it Chinese culture would not have developed enough to expand to the south and assimilate foreign conquerors.
Such an endorsement from the "Father of Modern China" started to transform the Great Wall into a national symbol in the Chinese consciousness, though this transformation was hampered by conflicting views of nationalism with regard to the nascent "new China.
"The failure of the new Republic of China fanned disillusionment with traditional Chinese culture and ushered in the New Culture Movement and the May Fourth Movement of the mid-1910s and 1920s that aimed to dislodge China's future trajectory from its past.
Naturally, the Great Wall of China came under attack as a symbol of the past.
An example, an influential writer of this period, Lu Xun, harshly criticized the "mighty and accursed Great Wall" in a short essay: "In reality, it has never served any purpose than to make countless workers labour to death in vain... surrounds everyone.
"The Great Wall itself suffered battle damage in the tumultuous Warlord Era of Republican China (191628).
The stretches near Shanhai Pass featured in both the First (1922) and Second Zhili–Fengtian War (1924), when the Manchuria-based Fengtian clique vied to control the civilian Beiyang Government against the Hebei-based Zhili clique.
While Fengtian forces overran the Shanhai Pass early in the First Zhili–Fengtian War, the Second saw the Zhili clique hold the pass until the Beijing Coup behind Zhili lines caused the defenders to waver and brought a swift conclusion to the war.
Sino-Japanese conflict (193145) gave the Great Wall a new lease of life in the eyes of the Chinese.
During the 1933 defence of the Great Wall, inadequately-equipped Chinese soldiers held off double their number of Japanese troop for several months.
Using the cover of the Great Wall, the Chinese – who were at times only armed with broadswords – were able to beat off a Japanese advance that had the support of aerial bombardment.
With the Chinese forces eventually overrun, the subsequent Tanggu Truce stipulated that the Great Wall was to become a demilitarized zone separating China and the newly created Japanese puppet state of Manchukuo.
Even so, the determined defence of the Great Wall made it a symbol of Chinese patriotism and the resoluteness of the Chinese people.
The Chinese Communist leader Mao Zedong picked up this symbol in his poetry during his "Long March" escaping from Kuomintang prosecution.
Near the end of the trek in 1935, Mao wrote the poem "Mount Liupan" that contains the well-known line that would be carved in stone along the Great Wall in the present day: "Those who fail to reach the Great Wall are not true men" ().
Another noteworthy reference to the Great Wall is in the song "March of the Volunteers" from the 1935 film "Sons and Daughters in a Time of Storm".
The song enjoyed continued popularity in China, and was selected as the national anthem of the newly established People's Republic of China (PRC) in 1949 when the communists emerged victorious from the Chinese Civil War.
In 1952, the scholar-turned-bureaucrat Guo Moruo laid out the first modern proposal to repair the Great Wall.
Five years later, the renovated Badaling became the first section to be opened to the public since the establishment of the PRC.
The Badaling Great Wall has since become a staple stop for foreign dignitaries who come to China, beginning with Nepali prime minister Bishweshwar Prasad Koirala in 1960, and most notably the American president Richard Nixon in his historic 1972 visit to China.
To date, Badaling is still the most visited stretch of the Great Wall.
Other stretches did not fare so well.
During the Cultural Revolution (196676), hundreds of kilometres of the Great Wall — already eroded by wind and rain — were deliberately destroyed by fervent Red Guards who regarded it as part of the Four Olds to be eradicated in the new China.
Quarrying machines and even dynamite were used to dismantle the Wall, and the pilfered materials were used for construction.
As China opened up in the 1980s, reformist leader Deng Xiaoping initiated the "Love our China and restore our Great Wall" campaign () to repair and preserve the Great Wall.
The Great Wall was designated as a UNESCO World Heritage Site in 1987.
However, while tourism boomed over the years, slipshod restoration methods have left sections of the Great Wall near Beijing "looking like a Hollywood set".
The less prominent stretches of the Great Wall did not get as much attention.
In 2002 the New York-based World Monuments Fund put the Great Wall on its list of the World's 100 Most Endangered Sites.
In 2003 the Chinese government began to enact laws to protect the Great Wall. 
 "The Great Wall of China" (1949)  
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The Hsiung-nu appear on the western borders of China.
The Han emperor Gaozu is defeated by the Xiongnu in the Battle of Baideng.
The Great Wall of China is completed.  
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