Romano Prodi in the context of The Democrats (Italy)

The centre-left coalition (Italian: coalizione di centro-sinistra) is a political alliance of political parties in Italy active under several forms and names since 1995, when The Olive Tree was formed under the leadership of Romano Prodi. The centre-left coalition has ruled the country for more than thirteen years between 1996 and 2021; to do so, it had mostly to rely on a big tent that went from the more radical left-wing, which had more weight between 1996 and 2008, to the political centre, which had more weight during the 2010s, and its main parties were also part of grand coalitions and national unity governments.

The coalition mostly competed with the centre-right coalition founded by Silvio Berlusconi. In the 1996 Italian general election, The Olive Tree consisted of the majority of both the left-wing Alliance of Progressives and the centrist Pact for Italy, the two losing coalitions in the 1994 Italian general election, the first under a system based primarily on first-past-the-post voting. In 2005, The Union was founded as a wider coalition to contest the 2006 Italian general election, which later collapsed due to Clemente Mastella during the 2008 Italian political crisis, with the fall of the second Prodi government.

Horst Siebert (20 March 1938 – 2 June 2009) was a German economist. He was a member of the German Council of Economic Experts from 1990 to 2003. Siebert also served as a member of both the Group of Economic Analysis (GEA) and the Group of Economic Policy Analysis (GEPA), a number of "European economists who advise the European Commission’s president." From 2002 to 2004, as a member of GEA, he advised EU President Romano Prodi. From 2005 to 2007, as a member of GEPA, he advised EU President Jose Manuel Barroso Siebert spent most of his academic career at the University of Kiel, where he held the chair for economic theory from 1989 to 2003.

A native of Neuwied, Rhineland-Palatinate, Siebert studied economics at the University of Cologne (1959–1963), while also studying, and undertaking research in, economics at Wesleyan University (1960–1961). He earned his doctorate degree in economics from the University of Münster in 1965.

The Olive Tree (Italian: L'Ulivo) was a denomination used for several successive centre-left political and electoral alliances of Italian political parties from 1995 to 2007.

The historical leader and ideologue of these coalitions was Romano Prodi, Professor of Economics and former leftist Christian Democrat, who invented the name and the symbol of The Olive Tree with Arturo Parisi in 1995. For the 2006 general election The Olive Tree was largely supplanted by a wider Prodi-led alliance called The Union, while The Olive Tree remained a smaller federation of parties which merged to form the Democratic Party in October 2007, which continues to be the lead party of an unnamed centre-left coalition.

The 2006 Italian general election was held on 9 and 10 April 2006. Romano Prodi, leader of the centre-left coalition The Union, narrowly defeated the incumbent Prime Minister Silvio Berlusconi, leader of the centre-right coalition House of Freedoms. Initial exit polls suggested a victory for Prodi, but the results narrowed as the count progressed. On 11 April 2006, Prodi declared victory; Berlusconi never conceded defeat and an ensuing dispute formed.

Preliminary results showed The Union leading the House of Freedoms in the Chamber of Deputies, with 340 seats to 277, thanks to obtaining a majority bonus (actual votes were distributed 49.81% to 49.74%). One more seat is allied with The Union (Aosta Valley) and 7 more seats in the foreign constituency. The House of Freedoms had secured a slight majority of Senate seats elected within Italy (155 seats to 154), but The Union won 4 of the 6 seats allocated to voters outside Italy, giving them control of both chambers.