Unit 7 covers global conflict from 1900, and its organising question is not "what happened in the wars" but why the political order that existed in 1900 collapsed, and what replaced it.

At the start of the century the West dominated the global political order. Over the following decades both land-based and maritime empires gave way to new states, and the CED names the mechanism as a combination of internal and external factors 内外因素.

• The older land-based Ottoman, Russian, and Qing empires collapsed under that combination - internal strain and pressure from outside at the same time.
• States around the world challenged the existing political and social order, and the CED's named case is the Mexican Revolution 墨西哥革命, which arose from political and economic inequality.

The examinable habit here is pairing. An answer that gives only internal weakness, or only external pressure, has given half the explanation.

The CED compresses the causes into one statement, and every element of it is worth marks:

• Imperialist expansion 帝国主义扩张 and competition for resources 资源竞争.
• Territorial and regional conflicts 领土与地区冲突.
• A flawed alliance system 结盟体系 that turned a local conflict into a general one.
• Intense nationalism 极端民族主义.

Notice the shape of the claim: the first three supply the tension, and the alliance system supplies the escalation 升级. That distinction - between what made conflict likely and what made it global - is the single most useful thing to hold from this topic.

World War I was the first total war 总体战. The CED lists the methods governments used, and they are not only military:

• Political propaganda 政治宣传, art, and media to sustain public support.
• Intensified forms of nationalism 强化的民族主义.
• Mobilisation of economies and populations 经济与人口动员 - conscription, war production, and the direction of labour.
• Recruitment and use of colonial subjects 殖民地臣民 as soldiers and labourers.

Alongside this, new military technology led to increased levels of wartime casualties. The technological point and the mobilisation point are separate marks, so give both.

Following World War I and the onset of the Great Depression 大萧条, governments began to take a more active role in economic life. That is the general claim; the CED then supplies contrasting cases of how far that role went.

In the Soviet Union 苏联, the government controlled the national economy through the Five Year Plans 五年计划, implementing repressive policies at significant cost to the population 人口代价 while achieving rapid industrialisation.

The general pattern that makes this comparable across cases: economic crisis increased the state's claim on the economy everywhere, but the extent 程度 differed enormously - from regulation and public works at one end to full state control at the other. Questions about the interwar economy almost always reward that spectrum.

Between the two world wars, Western and Japanese imperial states predominantly maintained control over colonial holdings, and in some cases gained new territory. The hopes for self-determination 民族自决 raised by the peace settlement were largely not met for colonised peoples.

This is the bridge topic: the tensions the settlement left unresolved feed directly into both the causes of the next war and the anti-imperial movements of Unit 8. Naming that link is what makes a continuity answer strong.

The CED again gives a compound cause, and each element is examinable:

• The unsustainable peace settlement 和约 after World War I.
• The global economic crisis 全球经济危机 produced by the Great Depression.
• Continued imperialist practices 帝国主义行径.
• The rise of fascism and extreme nationalism.

Compare this with the World War I list and a real pattern appears: economic pressure and nationalism are common to both, while the specific additions in 1939 are the failed settlement and the new ideological regimes. A comparison question wants exactly that structure - shared causes named, then the differences.

World War II was also a total war, and governments used the same broad categories of method: propaganda, art and media, intensified nationalism, and the mobilisation of economies and populations including colonial subjects.

What changed was the technology and its scale. New military technology and new tactics - including the atomic bomb 原子弹, fire-bombing, and the waging of total war - led to increased levels of casualties, including a far higher proportion of civilian 平民 deaths than in the previous conflict.

The examinable comparison is therefore: methods largely similar, scale and technology different. An answer that says only "both were total wars" has not compared them; an answer that says only "the second was more destructive" has not explained why.

This topic asks for causes and consequences of mass atrocities in the period from 1900 to the present. The CED's framing is that the rise of extremist groups in power 极端主义团体 led to the attempted destruction of specific populations, and it names the Holocaust 犹太人大屠杀 - the Nazi killing of Jewish people - as the principal case, alongside other twentieth-century genocides and mass killings.

Handled as history, the examinable content is the pattern: an extremist regime takes state power, defines a population as outside the political community, and uses the machinery of the modern state - bureaucracy, transport, communication - to carry out destruction on a scale earlier regimes could not achieve. That last point connects this topic to the technology theme running through the whole unit.

The unit's reasoning skill is causation, and specifically the relative significance 相对重要性 of causes. That word "relative" is the whole task: you are not listing causes, you are ranking them and defending the ranking.

Worked example (a real AP exam question). AP World short-answer questions in this unit ask, for example, to "Explain one similarity between the ways governments conducted World War I and World War II." A full-mark answer names the similarity and supports it with specifics from both: "Both wars were total wars in which governments mobilised entire economies and populations. In both, states used propaganda and media to sustain public support, directed industrial production toward war material, and recruited soldiers and labourers from their colonies." Notice the requirement: a similarity claim must be evidenced on both sides. Evidence from one war only cannot demonstrate a similarity.

• For the collapse of the Ottoman, Russian, and Qing empires, always pair internal strain with external pressure. One alone is half an answer.
• Separate what made World War I likely (imperial competition, nationalism, regional conflict) from what made it global (the alliance system).
• Total war has a checklist: propaganda, nationalism, economic mobilisation, colonial recruitment. Use it for both wars and the comparison writes itself.
• The interwar economy is a spectrum of state involvement, not a single policy. Say how far a state went, not just that it intervened.
• Comparison questions need evidence on both sides. A detail about only one case cannot support a similarity or a difference.
• "Relative significance" means rank and justify. A list of causes with no argument about which mattered most does not answer the question asked.