The Land War (Irish: Cogadh na Talún) was a period of agrarian agitation in rural Ireland (then wholly part of the United Kingdom) that began in 1879. It may refer specifically to the first and most intense period of agitation between 1879 and 1882, or include later outbreaks of agitation that periodically reignited until 1923, especially the 1886–1891 Plan of Campaign and the 1906–1909 Ranch War. The agitation was led by the Irish National Land League and its successors, the Irish National League and the United Irish League, and aimed to secure fair rent, free sale, and fixity of tenure for tenant farmers and ultimately peasant proprietorship of the land they worked.

From 1870, various governments introduced a series of Land Acts that granted many of the activists' demands. William O'Brien played a leading role in the 1902 Land Conference to pave the way for the most advanced social legislation in Ireland since the Union, the Land Purchase (Ireland) Act 1903. This Act set the conditions for the break-up of large estates by government-sponsored purchase.

Alongside the political and legal changes, the "Long Depression" affected rent yields and landlord-tenant relations across all of Europe from the 1870s to the 1890s.

The population of Ireland was overwhelmingly rural; in 1841, four-fifths of the population lived in hamlets smaller than 20 houses. This ratio declined over the century, but only due to emigration from rural areas and not from growth of the towns and cities. Land in Ireland was concentrated into relatively few hands, many of them absentee landlords. In 1870, 50% of the island was owned by 750 families. Between 1850 and 1870, landlords extracted £340 million in rent—far exceeding tax receipts for the same period—of which only 4–5% was reinvested. This led landlords to take on a role of non-productive managers within the island's overall economy. Conflict between landlords and tenants arose from opposing viewpoints on such issues as land consolidation, security of tenure, transition from tillage to grazing, and the role of the market. The Irish nationalist politician Isaac Butt opined that Catholic Irish being tenants was worse than "the heaviest yoke of feudal servitude".

The Devon Commission of 1843–44 found that various forms of tenant right were practiced throughout Ireland, not just in Ulster. There was a tension between English law, which protected the absolute property rights of the landlord, and Irish custom on the other hand in which the tenant enjoyed an "interest" in the property, which he could buy or sell. This "interest" could be as much as 4–6 years rent, which incoming tenants had to pay with capital that they might otherwise have spent on their own improvements. In the decades following the Great Famine, rises in agricultural prices were not matched by rent increases, leading to an increase in the tenant's stake in the farm, which may have risen to as much as 10–20 years of rent. The existence of tenant right was accepted by creditors who would extend loans with the tenant right as collateral.

During the Great Famine (1845–1849), the poorest cottiers and agricultural labourers died or were forced to emigrate, freeing up land that was purchased by larger farmers. In 1850, the Tenant Right League briefly dominated Irish politics with the demand for free sale, fixity of tenure, and fair rent. Although it never caught on with the poor smallholders in Connacht which it was intended to help, the League spurred the creation of the Independent Irish Party. In 1870, the Liberal Prime Minister William Ewart Gladstone pushed through the Landlord and Tenant (Ireland) Act 1870. The act actually increased agrarian tensions, as landlords attempted to evade provisions intended to protect departing tenants, while the tenants retaliated by setting up local Tenants' Defence Associations. One such the Route Tenants' Defence Association, was however hostile towards the League.^{[citation needed]}

Agrarian crimes were rising during the late 1870s, from 135 in 1875 to 236 two years later. At the same time emigration (which acted as a pressure valve for political tension) decreased by more than half. Nevertheless, as late as 1877 the areas which would be heavily affected by Land League agitation were completely calm, without any hint of what was to come. In 1878, the Irish-American Clan na Gael leader John Devoy offered Charles Stewart Parnell, then a rising star in the Irish Parliamentary Party (IPP), a deal which became known as the New Departure. As a result of this agreement, the physical-force and parliamentary wings of Irish nationalism agreed to work together on the land issue. This collaboration was cemented by a meeting on 1 June 1879 in Dublin between Devoy, Parnell, and Michael Davitt. It is disputed what was actually agreed to. Davitt maintained that there was no formal agreement, while Devoy claimed that the IPP had promised not to act against the Irish Republican Brotherhood (IRB) and made other concessions in exchange for Irish-American support.

The west of Ireland was hit by the 1879 famine, a combination of heavy rains, poor yields and low prices that brought widespread hunger and deprivation. Compounded by the reduction in opportunities for outside income, especially seasonal agricultural income in Great Britain, many smallholders were faced with hunger and unable to pay their rent. Some landlords offered rent abatement, while others refused on the grounds that their tenants were participating in anti-landlord agitation. Irish historian Paul Bew notes that five of the largest landlords in Connacht also refused to contribute any money to relief funds, despite collecting more than £80,000 annually in rent. According to historians such as William Vaughan and Phillip Bull, the serious agricultural recession combined with a unified nationalist leadership set the stage to produce a powerful and lasting popular movement.