A flag's meaning is not fixed by whoever designed it. It gets renegotiated by everyone who flies it afterwards, which is how a symbol that once united people can end up dividing them. The flags below have all been fought over, and in a few cases the fight changed the flag.
The Confederate flag
The most contentious flag controversy in the United States centers on the Confederate flag, originally the battle flag of the Confederate States of America during the Civil War. Its red background, blue cross and white stars flew over Confederate armies, and for some people it still represents the valor and sacrifice of Confederate soldiers. For many others it represents the Confederacy's defense of slavery and the racial oppression that followed.
After the war the flag largely disappeared from public view. It was revived in the mid-20th century by groups opposing the Civil Rights Movement, which turned it into an emblem of resistance to racial integration and cemented its association with white supremacy.
The argument has carried into this century. After the 2015 Charleston church shooting, where the perpetrator was pictured with the flag, calls for its removal from public spaces intensified, and many states and institutions took it out of official use. Its display by private individuals remains contested.
The Union Jack
The Union Jack, the national flag of the United Kingdom, has been argued over on questions of national identity and decolonization. It combines the crosses of the patron saints of England, Scotland and Ireland, and its current design dates to 1801, after the union of Great Britain and Ireland. As the British Empire expanded, it became the flag of colonial rule and flew over territories across the globe.
With decolonization after the Second World War, many former colonies replaced it. India, Kenya and Jamaica all adopted new flags reflecting their own cultural heritage and their plans for the future.
Within the UK the questions have not gone away. Scottish independence, Brexit and the status of Northern Ireland have all raised the issue of what the flag now means. If Scotland or Northern Ireland were to leave the UK, the Union Jack would need redesigning, which is a practical problem as well as a symbolic one.
Japan's Rising Sun flag
Japan's Rising Sun flag, a red sun with rays extending outward, carries a complicated history in East Asia. It was originally the ensign of the Imperial Japanese Navy and later became associated with Japan's militaristic expansion and its wartime atrocities during the Second World War. For many people in China, Korea and other countries that lived under Japanese occupation, it is a reminder of that period.
Japan retained the flag for its Self-Defense Forces after the war, which keeps the dispute alive. Many Japanese see it as a symbol of national pride and tradition, and its appearance at international events regularly draws protest from people who read it as imperialism.
Diplomatic dialogue and educational initiatives have tried to close that gap. The flag itself has not lost any of its charge, which is a reasonable measure of how long historical wounds last.
South Africa's flag
South Africa's flag runs the other way: a national flag that arrived as an act of reconciliation rather than a source of division. Under apartheid, the national flag stood for white minority rule and incorporated elements of the British Union Jack and the Dutch Tricolor, a fairly literal statement of who the country was run by and for.
With the end of apartheid and the establishment of a democratic government in 1994, South Africa wanted a flag that could carry unity and diversity at once. The result, designed by Frederick Brownell, uses a combination of colors drawn from different parts of the country's history and its intended future.
It was widely embraced as the flag of the "Rainbow Nation," and it is one of the clearer cases of a national symbol moving from divisive to unifying inside a single generation.
The American flag
The Stars and Stripes carries 13 stripes for the original colonies and 50 stars for the current states, and for a great many Americans it is a straightforward symbol of freedom and democracy. It has also been a fixture of protest, from the civil rights movement through anti-war demonstrations, whenever activists wanted to highlight the distance between the country's stated ideals and its conduct.
Flag burning and kneeling during the national anthem have both set off intense arguments about patriotism, free speech and the right to dissent. On the legal question the Supreme Court has ruled that flag desecration is protected under the First Amendment. The cultural argument continues without any comparable resolution.
The Turkish flag
Turkey's flag, a white star and crescent on red, sits at the intersection of national identity and secularism. The star and crescent go back to the Ottoman Empire, and their continued use links the modern republic to that imperial past. The flag also stands for the secular, nationalist ideals of Mustafa Kemal Atatürk, who founded the Turkish Republic and set out to build a modern secular state.
More recently, the resurgence of political Islam has reopened the question of religion's role in the state, and the flag ends up carrying both sides of it. Controversies tend to surface during periods of political turmoil, coups and protests especially, when rival factions invoke the same flag in support of very different visions of the country's future.
The European Union flag
The EU flag, a circle of 12 gold stars on blue, was adopted in 1985 to stand for unity, solidarity and harmony among member states. It flies alongside national flags and is a common sight at EU institutions and events.
It has also drawn opposition from people who see European integration as a threat to national sovereignty. Brexit made that tension concrete, with the flag becoming shorthand in the argument over national versus supranational governance.
As the EU works through economic disparities, migration and internal political divisions, the flag stays both an aspiration and a provocation, depending entirely on who is looking at it.