It didn't "consistently fail".

Northern Italy has one of the best standards of living in the world. People from around the world study and try to emulate aspects of Italy's educational and healthcare systems, but it difficult -- much of the advantages to be found in Italy have such complex and deep historical roots that they are difficult to replicate elsewhere.

Michael Porter's "The Competitive Advantage of Nations" long ago identified Northern Italy's economy and society as a "cluster" of institutions, technologies, geographies and social capital which are both rare and valuable. Porter offers case studies of the complex networks of competencies that make Italy a "world's best" producer in a range of internationally traded goods, and the complex histories of these arrangements.

In the "history" of the History Channel, everything is a war. Most of human life, though, isn't. If you could to pick a place in the world to be born in the last five centuries: Florence, Siena, Bologna, Padua -- not bad picks. Italy has been famously wealthy and a notably high investor in and producer of art, science and culture for many centuries-- producing cultural properties centuries ago which are globally respected and valued today. Global respect for culture is an enduring national asset that has a value that in many cases exceeds a transitory deference to military compulsion.

Over a billion people belong to the Roman Catholic religion -- the largest in the world-- an institution which has been globally "successful" with an historically Italian leadership and institutional culture. Which proved "stronger" in the end: the Pope's divisions or the Warsaw Pact's?

It is a real mistake to see history only through the prism of warfare.

​

See:

Michael Porter. "The Competitive Advantage of Nations" (Harvard Business Review: March-April 1990) [Porter wrote an expanded version of this article as a book . . . the article is online and free, the book isn't and isn't]

Richard A. Goldthwaite. Wealth and the Demand for Art in Italy, 1300–1600. Baltimore: Johns Hopkins University Press. 1993.

Helliwell, John F., and Robert D. Putnam. “Economic Growth and Social Capital in Italy.” Eastern Economic Journal, vol. 21, no. 3, 1995, pp. 295–307.

KLESTINEC, CYNTHIA. “A History of Anatomy Theaters in Sixteenth-Century Padua.” Journal of the History of Medicine and Allied Sciences, vol. 59, no. 3, 2004, pp. 375–412

BARTLETT, KENNETH R. “Worshipful Gentlemen of England: The ‘Studio’ of Padua and the Education of the English Gentry in the Sixteenth Century.” Renaissance and Reformation / Renaissance Et Réforme, vol. 6, no. 4, 1982, pp. 235–248.

[Italy has a particularly important role in the history of medical education]

I think u/amp1212 is correct to point out that we need to have some sense of what the words 'fail' and 'missions' in your question mean. Specifically, the spirit of your question seems to be "Why does the unified Italian state seem to have a troubled development into the sort of nineteenth-century projects like imperialism, colonialism and military success where nations like Germany, France and Britain excelled?" I agree with u/amp1212's general notion that gauging the relative success of a state on its ability to fight a war has some serious problems and Italy has certainly undergone a monumental transformation after the fall of fascism.

That said, as I read it, your question is seemingly about the successes or failures of the state, which are frequently gauged by warfare and, as you point out, in the nineteenth-century colonialism. This way of 'measuring' history as if it were a Paradox game has some problems but the historical details of your question remain interesting. Why does Italy struggle where, as the comparison is frequently made, Germany succeeded in establishing a state? This response is mostly addressing the foundation of the Italian state and largely stays in the nineteenth-century, since that sets up the difficulties faced in the twentieth-century well. (also, it is my area of expertise)

There are a dizzying list of reasons that lead to the uneven and fraught development. A lot of the reasons could be jotted up to politics. While the prophets of the Risorgimento subscribed to a broad sense of unification, the varied wildly in politics. Giuseppe Mazzini, who set up the Roman Republic in 1848 was a republican, desirous of a unified Italy with no monarchy. Many of his supporters and compatriots shared similar sentiments but not all. If you want some of these political details I have a thread I answered here.

Generally speaking, the years after the Risorgimento were devastatingly difficult for the young Italian State. It lacked the military strength to establish its own foreign policy on its own, making diplomatic relations vitally important for the praxis of nineteenth-century geopolitics. This is why Lazio, the province where Rome is located, was not annexed until nearly a decade after unification in 1861 - Italy's ally, Napoleon III of France, wanted an territorial Papal State and so Italy was incapable of annexing it without the allowance of its more powerful ally. The controversy about whether the new Italian state should annex Lazio, generally referred to as the Roman Question, was the overwhelming focus of Italian foreign policy in the opening decade of the new kingdom. Worse yet, once Rome was annexed, the Pope -- a king in his own right according to Catholic law -- was none too thrilled about losing so much of his authority based on the political rhetoric of a new bourgeois liberal elite. The result was essentially a culture war between the Italian state and the Catholic Church. Given that, aside from a small population of urban jews, nearly all Italians were Catholic, the vicar of Christ disavowing the state did not do much for the formation of a national identity. Pius IX, who was Pope at the time, entertained and eventually enforced the non expedit, a law that heavily discouraged Catholics from participating in Italian elections. As you can imagine, the legitimacy of a moderate kingdom, with a comparatively broad voting franchise having the foundations of its political legitimacy called into question by the majority of the populations penultimate religious leader did not exactly make the project of state-building any easier.

Furthermore, starting in 1873, an economic crisis rocked the foundations of the globe. This was arguably the first global financial disaster and it effected peasants in eastern and southern Europe arguably most severely. The poverty of the populations rural in Italy is difficult to overstate. I have read secondary sources (that do not unfortunately offer primary sources for this claim) that most peasants in Sicily would have access to meat twice a year: Christmas and Easter. Italy is rather varied regionally, but broadly speaking the peasantry would likely have little more than polenta and bread to eat at the hight of the anni neri or "black years". It is also at this time that vast swathes of Italians - usually men - were migrating to the Americas or elsewhere in Europe for better wages. The problem of mass emigration was not something that Italian political elites and there were figures who wanted to turn redirect these migrations to an Italian colonial project.

To say nothing for the dozens of geographical, colonial, and travel societies in Italy -- similar associations were not uncommon anywhere in Europe in the nineteenth-century -- there were also political figures who promoted an Italian colonial agenda. The securing of colonies in the nineteenth-century was as diplomatic an affair as acquiring territory on the continent, and Italy, as noted earlier, struggled to get that diplomatic ascension. I wrote up a bit about Italy's claim for Tunisia here. Of course the most famous colonial affair is the Crispi's Ethiopia campaign, ending in the battle of Adowah in 1898, an Italian defeat by an African army. The generally racist tones of the period hailed this military defeat to be devastating to Italian morale and national prestige. This ruined Crispi's political career and essentially prompted Italy to return to its own internal problems: budding anarchist, socialist, and Catholic movements that threatened the state.

In all, Italy struggled largely because the peninsula it faced deep political tensions, cultural and intellectual divisions, serious economic problems and overall struggled both militarily and diplomatically to see any foreign policy through. Though, to return to the framing of your question: to what degree we can historicize these as failures is deeply debatable. Ultimately thinking along the lines of a state prompts these events to seem like failures, but there are profound and excellent things to be said about imagining history well outside of such definitions which both overly-generalize and are simultaneously restrictive and unhelpful. The crisis of emigration was certainly a problem for the state, but because of it millions of peasants moved to infinitely better opportunities in North America (and then they brought pizza!). The failure of the Ethiopian war secured Ethiopia as a free empire of its own under an African king, Menelik II. Ultimately, one of the greatest failures of the Italian constitution -- the bedrock of the state -- prompted over two decades of dictatorship, yet that detail is often overlooked for low-hanging jokes about Italian military performance in the twentieth century. As such, Italian historians can be a bit touchy about how these questions are framed. I hope this was helpful.

For more reading (specifically if you are interested in war, colonialism and society):

Arnold Blumberg, A Carefully Planned Accident: the Italian War of 1859, (London, 1990)

Raymond Jonas, The Battle of Adwa: African Victory in the Age of Empire, (Cambridge, 2011)

Alexander De Grand, Italian Fascism: It's Origins and Development, (Lincoln, 1982)

Giuseppe Finaldi, History of Italian Colonialism: Europe's Last Empire (1860-1907), (New York, 2018)