In my three years forecasting with Metaculus, I have predicted on over one thousand questions. Many of these questions cover the same topics. Rather than forecast each question on an ad hoc basis, I develop guides for particular domains.

My Forecast Factors Series aims to share my own guidelines and forecasts to support more accurate aggregate predictions by the Metaculus community.

This is a companion piece to Forecasting Civil Wars in the Modern World. I recommend reading it first for context. I also recommend reading some of the other, more scholarly articles that have been put out. At the beginning of this article, I will be looking at the factors that predict interstate war. Then I will use those factors to make predictions on a selection of notable potential conflicts.

As mentioned in the introduction of that article, interstate wars are considered harder to predict than civil wars since the decision to launch them lies in the hands of a few high-level players. Many of the comments under this question had Metaculites guessing what Russian President Vladimir Putin’s mental state was.

Interstate wars often begin with little notice in other ways. Few could have predicted that the assassination of the Austrian Archduke by Bosnian Serb terrorists would have led to World War I. Nor could many have predicted that Argentina would have attempted to take the Falkland Islands by force after 160 years of mild protest. Therefore it would be instructive for our purposes to have many warning signs to look for.

In my time researching this topic, I have been able to identify nine major factors that appear to most contribute to the risk of interstate wars. Before discussing these variables, it would be useful to show how often interstate wars occur. 

As with my civil war base rate, I will use the end of World War II as my starting point. I will then determine the number of interstate wars fought since that date. An interstate war will be defined as the forces of at least one country’s military losing at least 0.01% of its population from combat against another country’s military in the timespan of ten years. 

To be defined as a country, a belligerent must have recognition by at least one UN member. A country must also have a population of at least 800,000 at any point to be counted. Interventions in civil wars where the opposing side is not a de facto government, such as the Soviet intervention in Hungary or the Soviet-Afghan War, will not count. The Korean and Vietnam War will count, since both sides were de facto governments.

After searching through Wikipedia and the World Bank’s population estimates, I found that there have been 60 interstate wars since World War II. I'll now share nine factors that I identify as increasing the probability of interstate wars, in order of their importance.

Factors

1. The various wars between Israel and Lebanon, the First Nagorno-Karabakh War and the Russo-Georgian War are the only wars where both belligerents scored at least a 0.3 on Our World in Data’s Electoral Democracy Index. It should also be noted that Azerbaijan and Russia soon fell below 0.3 after the wars. This is extremely strong evidence backing up the Democratic Peace Theory.
2. In my own judgement, I identified land disputes as being the primary cause of 47 of 60 interstate wars. Of the remaining wars, most involved a great power wanting to flex its muscles or institute regime change, such as the Sino-Vietnamese War or the Iraq War. Outstanding land disputes should be taken as a warning sign of potential conflict.
3. There seems to be a correlation between the percentage of GDP spent on defense and the likelihood of a nation getting involved in an interstate conflict. The United States and Russia have had above average military spending for many decades and fought many interstate conflicts in that time. Other examples of high spenders who were primary actors in interstate wars include India, Pakistan, North Korea, South Korea, Greece, Turkey, Indonesia, Tanzania, Iran, Iraq, Argentina, Armenia, Azerbaijan, Rwanda, Ethiopia and Eritrea. In fact, only El Salvador, Honduras and Somalia had below average military spending before starting an interstate conflict.
4. Of the 60 interstate wars I examined, 27 were part of a series of wars between the same two opponents. Notably however, none occurred after more than 25 years of peace between the belligerents. 
5. Recent violent regime changes seem to be a warning sign of war. 28 wars occurred after one of the belligerent sides had experienced a violent coup, revolution or civil war in the previous 10 years.
6. Similar but distinct are countries that recently gained independence peacefully. Five wars were fought involving at least one country that had gained independence less than a year prior. These were the 1947 Indo-Pakistani War, the 1948 Arab-Israeli War, the Indonesian Invasion of East Timor, the First Nagorno-Karabakh War and Croatia’s involvement in the Bosnian War. In addition, Russia invaded Georgia and Ukraine after just 17 and 31 years of independence from them, respectively.
7. Of the 19 interstate wars fought since 1990, only five have been fought between two countries who both ranked in the top half of the Human Development Index. There are the United States invasion of Panama, the Croatian involvement in the Bosnian War, the various Armenia-Azerbaijan conflicts, the Russo-Georgian War and the Russo-Ukrainian War. While there is a correlation here, it is not as strong.
8. Multilateral military alliances are a strong deterrent against war between their member states. The only wars between allies in the data set are the Bay of Pigs Invasion, the Warsaw Pact Invasion of Czechoslovakia, the Football War, the Greco-Turkish War in Cyprus, the US invasion of Panama and the US invasion of Haiti.
9. There have been no great power wars since the Korean War. This is hard to disentangle from the fact that there have been no wars between countries with nuclear weapons either, since most great powers currently have nuclear weapons. There have been several close calls, however, such as the Cuban Missile Crisis and the Sino-Soviet border conflict.

Forecasts

Will China launch a full-scale invasion of Taiwan by the following years?

Writing this essay prompted me to update downward to 15% for the 2035 date, 9% for 2030, and 2% for 2025

I believe that the community is seeing signs that aren’t there. While China is very authoritarian and there is a major outstanding land dispute with Taiwan (the whole island!), there are more mitigating factors. 

China spends a below average and shrinking percentage of its budget on defense. It has not fought a war since 1979. It hasn’t fought against Taiwan specifically since the 1950s, which has allowed the issue to run cold. It ranks highly in human development and might be focusing on economic issues. The unique situation around Taiwan would likely drag in its largest economic partner, which also happens to be the most powerful country on Earth and also possesses nuclear weapons. Both the Taiwanese and Chinese regimes seem stable and unlikely to provoke a conflict.

I am now at 2% for 2025 and 12% for 2030.

There are several reasons to fear conflict in the short term. Venezuela is not only heavily authoritarian, but the regime is under threat and could use a distraction. Its economy is in tatters and Maduro has nothing to lose. The Essequibo dispute is long-running, but it has no recent history of violence. The Rio Pact has proven ineffective in the past.

However, Venezuela spends a paltry amount of its GDP on defense. The US might intervene and Venezuela would likely face a severe loss the same way Argentina did at the hands of the UK. It should be noted though that unlike China, Venezuela lacks a nuclear weapons program, so it has no reason to fear escalation in that battlespace.

I am at 4% for 2025 and 22% for 2030.

Azerbaijan continues to undergo democratic backsliding, which likely contributed to Aliyev feeling secure enough to launch the very recent Second Nagorno-Karabakh War. Aliyev has used language suggesting that the state of Armenia is illegitimate. Both countries continue to spend a high percentage of their GDP on defense. Russia has already proved an ineffective ally for Armenia to prevent further Azeri attacks.

The only mitigating factor I can think of are the ongoing peace talks.

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Potential Issues

It is far more difficult to establish a base rate for how often one should expect the average country to get involved in any interstate war. This is due to the difficulty in determining who fully counts as a belligerent in an interstate war. Personally, I used a base rate of 2% per annum after looking at a large sample size of countries, but this is subject to far more conjecture than my civil war base rate.

Many of my factors use indexes that are run by Western think tanks. Therefore their potential bias should be taken into account.