Ethiopian Prime Minister Abiy Ahmed ordered troops into the northern region of Tigray on Nov. 4, accusing a powerful faction of traitorous behavior. On Saturday, the government claimed its military took control of the region’s capital city, Mekele. On Monday, the Tigray leadership accused Ethiopian troops of launching a “genocidal campaign” in the region.

Tensions have been brewing with the Tigray People’s Liberation Front (TPLF), once a dominant force in Ethiopia’s regime, since Abiy gained power. The conflict erupted soon after Abiy claimed that the TPLF crossed a “red line” when Tigrayans attacked a federal military base in early November. Fears of mass atrocities grew after Abiy’s 72-hour ultimatum for the TPLF’s surrender expired last week.

Could Abiy’s offensive turn into a broader civil war, and draw in neighboring countries? Tens of thousands of civilians have fled into Sudan, threatening its stability. And earlier this month, Tigray forces reportedly fired multiple rockets into Eritrea, a long-standing rival of the TPLF. If Sudan or Eritrea decides to intervene, other outsiders may follow.

My research explains why foreign interference in Ethiopia’s conflict is predictable — and why interventions by external militaries tend to make bad situations worse.

Civil wars are rarely purely domestic affairs

Foreigners often meddle in the internal battles of other countries — and outsiders are getting increasingly meddlesome. The data reveal that outside interventions in civil wars have increased in recent years, both in terms of absolute numbers and in the proportion of ongoing wars.

During the Cold War, most interventions were shaped by the U.S.-Soviet rivalry. After the fall of the Soviet Union, countries focused on finding negotiated settlements and deploying peacekeeping forces to keep the peace. But the United States, regional powers and neighboring countries became less hesitant to intervene after Sept. 11, 2001, and as the U.S.-led liberal order began showing signs of decay.

Most full-fledged civil wars today include foreign militaries, groups and fighters.

Why do foreign militaries intervene?

Foreign militaries are not innocent bystanders to civil wars. In fact, internal conflicts attracted external militaries last year more often than at any time since World War II. We may have been underestimating the extent of external involvement in African conflicts in particular.

One reason for these interventions is because the ill effects of a localized conflict are never neatly contained within the confines of a war-torn country. Refugees, rebels and weapons — along with drugs and other illicit materials — can spread easily beyond national borders, threatening the stability of nearby countries.

A record number of refugees are living abroad — the highest totals in the post-World War II period — as recent civil wars displace greater numbers than in previous decades. Large numbers of refugees pose risks to weaker host countries. The strain on financial resources, threats to internal security, shifts in local demographics or new animosities among populations can create incentives for intervention.

Tigray absorbed an estimated 100,000 Eritreans from a previous war, many of whom could be displaced yet again. The rapid pace of Ethiopians now arriving in Sudan reportedly is overwhelming aid organizations — and the numbers seeking safety are likely to continue to climb. Sudan itself is in the middle of a fragile transition after its longtime dictator was toppled last year, and is poorly placed to absorb the influx.

Conflicts also affect economies in neighboring countries. Research indicates that the negative economic effects can stretch hundreds of miles outside the country at war, with losses in regional investment and increases in illegal trade. So neighbors care about conflicts in their own backyard.

Civil wars heighten the risk of wars between nations

Analysts argue that one of the best predictors of civil war is if there is another civil war in the region. The negative transnational effects of wars can aggravate domestic tensions and spark internal conflicts in other countries. Conflicts spread and contagion happens.

External militaries intervening to prevent negative spillovers can also instigate a broader international dispute. Likewise, regimes waging domestic battles may see a reason to conduct cross-border counterinsurgency campaigns or attack foreign rivals that are supporting opposition groups. Political scientist Idean Salehyan finds that more than half of all rebel groups rely on external sanctuaries during civil wars, so governments often want to chase down their enemies abroad.

But outsiders tend to mess things up

When external military forces get involved, they often harm internal and international security. In Syria and Yemen, for instance, the reality that so many outsiders are involved in these conflicts means that fighting is unlikely to end anytime soon.

Interventions typically provoke additional interventions by rival countries. And once there are several competing foreign forces involved, civil wars tend to last longer, the research shows. It’s simply harder to find a compromise that makes enough people happy during negotiations — potential solutions are complicated by more parties with unique interests.

If parties agree to peace, interventions make conflicts more likely to recur again — that was the case in Afghanistan, Congo and Somalia.

Local struggles can escalate rapidly into proxy wars, when a major power benefits by having others fight a conflict. Take the war in Libya, for example. The sheer number of outsiders — Egypt, France, Jordan, Qatar, Russia, Sudan, Saudi Arabia, Turkey, the United Arab Emirates and the United States — has transformed the conflict into something quite different from when it started. Proxy wars often drag on, and it’s always possible for them to escalate into wider conflicts.

To be sure, not all outsiders make things worse. Most studies find U.N. peacekeepers effective, though of course far from perfect — their presence tends to reduce civilian deaths, shorten wars, prolong peace and diminish the diffusion of conflict to other countries. When outside military forces get involved in a country’s internal conflict, however, the same results do not occur.

In short, interventions tend to provoke more interventions, prolong civil wars and increase the chances of war between countries. None of this bodes well for Ethiopia. Neighboring Eritrea, Sudan, Egypt and other countries could become key players in an all-out civil war with no clear end in sight.

David Kampf (@davekampf) is a senior PhD fellow at the Center for Strategic Studies at the Fletcher School.