"Iran is a serious adversary". More so than North Korea, Russia, China? Of the ones mentioned the most serious threat is from?

Iran is a major destabilizing influence for that region, as well as other areas, as they pose a threat to commerce in the Strait of Hormuz. They are sponsored, backed, encouraged and covered by Russia, the technical adviser to the nuclear program now in progress there. The Russians are the big players on the block in the Middle East and will remain so as Biblical prophecy unfolds and becomes fulfilled.

I'd say China, but not militarily if it can get its way without warfare - which would hurt it as badly as it hurts others.

Iran and its backers are a threat to stability in the Middle East, but Israel's not far behind and the running fight between the Palestinian and Israel governments and their supporters, is a major cause of much of the Middle Eastern instability. Remember, "Israel" was foisted by the UN onto the region in the wake of what the Nazis had done to the Jews in 1930s-40s Europe; and the UK was a leading partner in forming Israel both from having born much of the brunt of defeating Hitler, and as Palestine was Arabic but British-controlled territory. Until then, most of the residents and representing between them all of the three major religions invented in that [Un]Holy Land had co-existed fairly well.   

Groups like ISIS exploit a nasty mix of territorial and sectarian ambitions, while the fighting in the Yemen is equally exploited by larger, external powers. As ever, the wars are led by small groups of vain, egotistical men pushing their own religious/political/ territorial/ economic ambitions; while the country's citizens at large suffer the consequences. 

The Russian Federation as run by President Putin seems to want the power once held by the USSR. The USA as run by President Trump still thinks it can be both isolationist and tell the rest of the world how to behave, but whilst promoting "regime change" can't say to what the regime might change.

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Meanwhile the People's Republic of China takes its slow, patient, subtle time; building up its economic power over as much of the world as it can - unfortunately blindly helped the West. For example: Italy supporting massive transport projects owned by China; the UK possibly wanting Huawei to build much of the next portable-'phone network; major companies in many Western nations including the USA moving production of goods to the lower-cost China.

North Korea? Too small to do much damage except regionally and as an ally of China; but it largely kept itself to itself until the US tried to open disarmament discussions with its fickle, paranoic leaders. 

These two Communist states and the Russian Federation have a potentially much more powerful weapon than nuclear bombs in their arsenals, too: control of the Internet and by it, so much of other nations' and people's lives and livelihoods. The Internet's inventor, Tim Berners-Lee, envisaged it as an open, democratic exchange system, but it has largely been taken over by a few huge, remote and unaccountable American corporations. Thinking as Devil's Advocate, those other State might want that situation stand both for its direct technical support, and for the technical monopoly aiding their own governmental and commercial eavesdroppers and hackers.  
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Finally, this has nothing to do with vague Biblical "prophecies" by people who knew nothing about the world outside their region. It may be easy to believe in ancient fortune-tellers; but we ought reflect that the Bible expresses the Hebrews' own view unto themselves, of wanting a land to call home in their own time without worrying whose land it was or of consequences even then, let alone two millennia hence. 

Thank you Rosie!

I am afraid international politics is far more complicated and difficult than it often seems. There is not one ball in any one court, but many balls all over the place, all in play at once.

As for who are the brightest leaders, I hate to say. Most of them are very intelligent people but they are pushing their own or their countries' interests above all, and we never really know who has the real power behind many of them.

I wonder if we will ever know the truth behind that stopped air attack. This is the second time Trump has backtracked on something affecting other countries, in a couple of weeks, so whether he is beginning to learn something about international affairs, or his advisors are becoming discreetly stronger, is anyone's guess.

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It has occurred to me that Britain and the other signatories still maintaining the nuclear-power deal with Iran ought say to the Iranians,

"We are still in that deal, we are not threatening to attack you. We suspect you in the attacks on the oil-tankers, but if you are innocent then why not let us help you find out who is?"

(I wonder if it's actually ex-ISIS or similar people, wanting Iran blamed so they aren't, but mainly to encourage trouble in the Middle East for their own ends.)