War, revolution, industrialisation and growing tensions between Germans and Czechs characterised the 19th century in Iglau. In 1805, in the run-up to the Battle of the Three Emperors at Austerlitz (Slavkov), French troops marched into the town; during the Prussian–Austrian War of 1866, the Prussians occupied Iglau for two months. Although the Revolution of 1848–49 led to the formation of a National Guard, it otherwise left little mark.

Mayor Peter Ernst Leupold von Löwenthal (1800–1872), who held office from 1850 to 1857 and from 1863 to 1871, rendered great services to the modernisation of the town. With the new municipal code of 1850, decision-making powers were transferred from the magistrate to the elected representatives of the local government. Iglau had been the seat of a district administration since 1849.

The intensifying tensions between Czechs and Germans led, in 1910, to violent clashes between the two ethnic groups following a meeting of the Czech gymnastics association Sokol; these events became known as the ‘Iglau Blood Chain’. On 3 November 1918, the German members of the Austrian Imperial Council advocated the annexation of 79 settlements around Iglau – referred to as a ‘linguistic enclave’ – to German Austria; but following the First World War, the area of approximately 770 km² was ceded to the newly founded Czechoslovakia under the Treaty of Saint-Germain in 1919.

On 5 December 1918, the town hall, which had been under German control, was forced to bow to the new circumstances of the First Republic. A government commissioner initially took charge of the town administration, assisted in an advisory capacity by a commission composed of an equal number of Czechs and Germans – twelve of each. In the local elections of 28 September 1919, the German Social Democratic Party won almost 29 per cent of the vote; however, following numerous objections, the elections were annulled in their entirety by the Provincial Political Administration in Brno and repeated a year later. Once again, the German Social Democrats emerged victorious, closely followed by the German Democratic Party and the Czechoslovak Social Democrats. By drawing of lots, the merchant Othmar Oberrenner (1871–1936) was chosen as the representative of the German Democrats and became the last German mayor; he was succeeded in 1923 by the schoolteacher Josef Výborný (1889–1982) as government commissioner in 1923; during his term of office, which lasted just under two years, German street names were renamed in Czech, Czech was granted full equality as the language of official business, and the Czech school system experienced a significant upswing. In 1925, the Social Democrat and lawyer Rudolf Veverka (1880–1949) became the first Czech to head the municipal council. In 1928, a monument to Hus was ceremonially unveiled to mark the tenth anniversary of the First Republic; however, in the same year, Iglau lost its status as a statutory town – a status it had held for almost six decades – following a government decree.

In the 1929 local elections (the last ones until 1938), the Czech Social Democrats once again won by a wide margin (approx. 25 per cent); of the six German parties, the German Labour and Economic Association secured the most votes (8.4 per cent). The delayed effects of the 1929 global economic crisis (a soup kitchen had been in operation since November 1932), the external and internal political threat posed by fascism (the German Homeland Party, founded in 1924 as an offshoot of the DNSAP; a branch of the Národní obec fašistická established in 1926) as well as the increasingly active role of the SdP (which won 36 per cent of the vote in the 1938 local council elections) overshadowed events far beyond Iglau, such as the 500th-anniversary celebrations of the Basel (Iglau) Compactates in 1936.

With the establishment of the so-called Protectorate of Bohemia and Moravia , German troops also marched into Iglau on 15 March 1939; the town subsequently became the seat of an Oberlandrat (district council) tasked with rapid Germanisation, and the occupying forces also set up a branch office here of the Brno Gestapo and the Criminal Police. Street renamings in 1940 (including Masarykovo nám. to Adolf-Hitler-Platz) were intended to emphasise the ‘Germanic’ character of Iglau, even though in 1940 there were 12,477 Germans and 17,727 Czechs living in Iglau. Jewish property fell victim to ‘Aryanisation’.

On 9 May 1945, the Red Army entered Iglau without a fight; the German population fell victim to violent expulsions and forced resettlement. The Germans from the villages ofthe ‘language enclave’were also expelled in July 1945; some were sent to internment camps or forced into labour.

Around the middle of the 19th century, further factories were established around Iglau, including, in 1847, the cloth factories of Jakob Kern (1790–1867) in Staré Hory / Altenberg and of Adolf Löw (1824–1883) in Helenenthal/Helenín (1860) and Klein Beranau/Malý Beranov (1870). Theodor Kern (c. 1858–1919) turned the ‘Altenberg Military Cloth and Sheep’s Wool Goods Factory Enoch Kern & Sons’ into one of the most important suppliers to the Austro-Austro-Hungarian Army, supplying a third of its military requirements between 1870 and 1874. By the early 20th century, the company was selling its goods worldwide. The Löw family belonged to the entrepreneurial elite of the entire monarchy and rendered great services to the modernisation of production. Karl Löw (1849–1930) was one of the first entrepreneurs to introduce health insurance for workers in his company and had several workers’ housing estates built for them.

Today, mechanical engineering and the timber industry in particular play an important role.

Against the backdrop of escalating ethnic and national conflicts in the town, Iglau became an early centre of the German Workers’ Party (DAP, known as the DNSAP from 1918), which was founded in 1903, which Adolf Hitler later described as one of the first signs of life of the Nazi movement in Central Europe. It found a political theorist in the railway engineer Rudolf Jung (1882–1945)—who attended secondary school in Iglau, served as a DAP town councillor there before the First World War, and is regarded as a co-author of the DAP’s ‘Iglau Programme’—as well as in Hans Krebs (1888–1947), the son of an Iglau cloth maker and innkeeper, and chief executive of the DNSAP in Czechoslovakia from 1918 to 1931. Both were fanatical admirers of Hitler and, from 1933 and 1939 respectively, held high political offices in the Nazi state and in the Protectorate of Bohemia and Moravia.

Alongside the political oppression and persecution of the Czech population between 1939 and 1945, there were also various forms of collaboration with the German occupiers, motivated by different factors. František (Franz) Löffler (*1898), a native of the Czech Republic, founded the ‘Národněsocialistická garda slovanských aktivistů’ (NSGSA) [National Socialist Guard of Slavic Activists] in Iglau and acquired German nationality in 1943. After the end of the war, the Iglau Extraordinary People’s Court sentenced him to life imprisonment. He was granted an amnesty in 1955. In addition, since March 1939, there had been numerous resistance groups in Iglau and the surrounding area opposing the German occupiers, whose activities ranged from distributing leaflets to – particularly in the final phase of the war – carrying out bomb attacks (10 April 1945: the demolition of the strategically important railway bridge near Helenenthal).

Alongside the Church of St John the Baptist in Old Iglau, the oldest place of worship, the parish church of St James in the east of the town, consecrated in 1257, played an important role, as did the Minorite Church of the Assumption, built around 1245 in the west of Iglau, and the Dominican monastery with the Church of the Holy Cross, situated north-east of the market square.

During the Hussite Wars, the town, with its conservative German upper class, formed a bulwark of the Catholic faith. From 1423 onwards, Hussite armies tried in vain to capture the well-fortified stronghold. In early July 1436, the Hussite Wars officially came to an end in Iglau when, in the presence of Emperor Sigismund of Luxembourg (1368–1437), legates of the Council of Basel and representatives of moderate Hussites, the treaty agreement (the Compactata) reached in Basel on matters of faith was sworn to and publicly read out.

As early as 1522, the Reformation made its way to Iglau in the person of Paulus Speratus (1484–1551), but the former follower of Luther was soon expelled from the country at the instigation of the Bishop of Olomouc. The defeat of the Protestant Bohemian estates at the Battle of White Mountain was followed by a systematic return to Catholicism in Iglau as well. Until the dissolution of their order in 1771, the Jesuits ensured the consolidation of the Catholic faith and founded religious confraternities (in 1628, the Latin Confraternity of the Visitation of St Elizabeth; in 1635, the Confraternity of the Assumption of Mary). In 1628, the Capuchins also settled in the town.

Jews firstappeared in Iglau in the mid-13th century, and in greater numbers in 1345, when Charles IV, for economic reasons, encouraged their settlement in Judengasse in the western part of the town, where they also built their synagogue. In 1425, however , the Jews were forced to leave the town, relinquishing their property and waiving their claims to debts. From then on, the synagogue served as a hospital. It was not until the end of the 18th century that a few privileged Jewish families were once again able to take up residence in the town. A larger influx took place after 1848 in the wake of increasing industrialisation. The synagogue was consecrated in 1863. From an initial number of 17 Jews ( 1837), their number rose to 1,415 by 1880, by 1938 there were still 700 Jews living in Iglau. The Iglau Jews who remained in the Protectorate of Bohemia and Moravia were systematically deprived of their rights, marginalised and finally, after the synagogue had already been set alight by the German occupiers on the night of 29–30 March 1939, were systematically stripped of their rights, marginalised and finally, from 1941 onwards, deported to German concentration and extermination camps. Only 18 Jews from Iglau survived the Holocaust; a memorial has commemorated this since 1995.

In 1561, a humanist grammar school was founded, at which graduates of the universities of Wittenberg and Frankfurt/Oder taught . The fact that town clerks also distinguished themselves as town chroniclers is illustrated by the example of Martin Leupold von Löwenthal (1556–1624), whose comprehensive chronicle of Iglau covering the years 799–1619 remains of great significance to this day, thanks to its inclusion of transcribed documents and the accuracy of its records.

A Czech grammar school had been in existence since 1882; between 1889 and 1890, the building housing the German grammar school, complete with a library, was erected. The town museum opened in 1892.

Notable figures born in Iglau include, amongst others, Carl Friedrich Freiherr von Kübeck (1780–1855), who was a strong advocate for the expansion of the railway and telegraph networks; the writer Karl Hans Strobl (1874–1946), and the poet and playwright Louis Fürnberg (1909–1957), who moved to the GDR in 1953 in the wake of the Stalinist-orchestrated Slánský trials.

Since 2004, Iglau has been home to a polytechnic university with around 600 students.