As the number of adversaries that Israel is fighting has piled up over the past year — Hamas in Gaza, the Houthis in Yemen and Hezbollah in Lebanon — the most worrisome prospect has been the potential for a war with Iran.

The two nations, which do not share a border, have long been waging conflicts by proxy, subterfuge and sabotage. Each of the militant groups Israel is concurrently fighting is backed by Iran. The indirectness was always by design: Despite being regional rivals, each wanted to avoid what was sure to be a costly, existential direct confrontation.

Now, with Israel planning a retaliatory attack after Iran’s ballistic missile barrage of unprecedented scale and scope on Oct. 1, a war seems more likely, alarming the international community and countries in the region.

Here is why a war is so concerning.

Israel and Iran have two of the region’s most formidable militaries.

Israel has one of the world’s most technologically advanced militaries and is among the top military spenders globally as a share of gross domestic product. Israel’s arms industry produces weapons at such a high capacity that last year it produced enough to export a record amount despite its war in Gaza, according to researchers. Israel is also heavily backed by the United States, which has supplied more than 29,000 guided bombs, artillery rockets and assorted missiles since 2009.

Iran’s armed forces are among the largest in the Middle East, with at least 580,000 active-duty personnel and about 200,000 reservists, according to an assessment last year by the International Institute for Strategic Studies. Iran has made the development of precision and long-range missiles a priority for decades and has amassed one of the largest arsenals of ballistic missiles and drones in the region. The country also has a sizable inventory of drones, with ranges of up to 1,550 miles and the ability to fly low to evade radar.