The TV debate between Kamala Harris and Donald Trump was as keenly watched by European diplomats and politicians as by US voters, eager to see who may be next in the White House and – crucially – the direction that a vital ally may next take.

One diplomat said they empathised when Harris adopted a series of poses that ranged from pity, bemusement and genuine curiosity about what craziness would emerge from Trump’s mouth next as she listened to his conspiracy-laden theories. However, the diplomat said they still did not underestimate Trump and the hold he had over one part of a divided America, adding: “Never write him off.”

Another European observer judged Harris to have been the victor in the debate. “Objectively on any count she won. She showed her teeth, broke with [Joe] Biden and showed she is a leader, and that is something Americans love,” they said.

In Germany, Michael Roth, the Social Democratic party chair of the Bundestag foreign affairs committee, said Harris had succeeded in making Trump seem “like an ageing incumbent, old, angry and confused”, despite having been in government herself for much of the past four years. “Harris has dismantled Trump on the open stage and positioned herself as a candidate of change. She deliberately provoked Trump, and he fell into the trap,” Roth said.

The German foreign office, in a tweet, hit back at Trump’s criticism of German energy reliance on Russia writing: “Like it or not: Germany’s energy system is fully operational, with more than 50% renewables. And we are shutting down – not building – coal & nuclear plants. Coal will be off the grid by 2038 at the latest. PS: We also don’t eat cats and dogs.”

However, above all, the 90-minute debate served to underline how a Trump victory would threaten the fundamental pillars of European security.

Diplomats were most struck, if not surprised, by Trump’s refusal to say whether he wanted Ukraine to defeat Vladimir Putin. During the debate, Trump said he only wanted the war to stop and for lives to be saved, something he said he would achieve in days through direct talks with both sides.

Trump’s praise for the “tough, smart and respected” Hungarian prime minister, Viktor Orbán, also served to underline the Republican nominee’s close alignment with the surging populist right in Europe rather than its mainstream leaders, many of whom have been politically weakened by the anti-immigration rhetoric that the former US president deploys.

“Ironically, I don’t think this alliance will help Orbán or Trump,” one diplomat said. “It just shows how disconnected Trump is. No one in the US knows who this Hungarian politician is.”

Trump’s claims about what he had done to invigorate European spending in Ukraine were familiar if statistically false.

His assertion that Israel would be destroyed within two years if Harris was elected only served to underline how he might bring a diplomatic blowtorch to a region already close to explosion. Without explaining, Trump said Harris hated Israelis and Arabs, and claimed: “The whole place is going to get blown up.”

Harris said too many Palestinians had died in Gaza and a two-state solution was needed, something Trump has recently pulled away from.

Trump’s reference to how he had threatened the Taliban leader Abdul – possibly a reference to Abdul Ghani Baradar, the Afghan first deputy prime minister – by sending a picture of his house also revealed his diplomatic methods.

On Iran, Trump left diplomats puzzled by accusing Biden of allowing the Iranian oil industry off the hook in terms of sanctions, having said only days earlier that Tehran posed no threat to US democracy.

Trump’s contradictory positions seem to be driven by a desire to appear a tough leader in a world on the brink of chaos, yet also knowing many Americans believe US foreign policy has involved too many expensive and unproductive wars. Polls conducted by the libertarian thinktank the Cato Institute, for example, suggest voters in swing states back his promises to end US overseas commitments.

Before the debate on Tuesday night, European diplomats conceded they had been tenterhooks, hoping Harris would be able to recover her summer momentum by performing well in the TV head-to-head.

One admitted that the first numbers to which they referred each morning were those in US opinion polls. The contradictory polling results did not help with the digestion of breakfast, the diplomat added.

They pointed out that if Europe was concerned by Biden’s performance in his faltering June debate, and his subsequent slide in the polls, there was at least the exculpatory comfort that US voters were responding logically to the decline in the president’s faculties.

The fact that Trump had edged back into the lead in some polls two months on from Biden standing aside, and with just 55 days to go before the US presidential election, suggested America had reverted to “an angry type”, the diplomat added.

For many European diplomats it was telling that in the US today, faced with the choice of either a misogynistic, rambling and divisive Republican or a sane and credible Democrat candidate, voters still remained unsure which one should lead them.

The preference for Trump over an ageing Biden was explicable, but the recent stalling of the Harris campaign pointed to deeper forces at work, diplomats said. It suggested the US wanted change, and Harris, as vice-president, had not yet done or said enough to convince voters she offered that. One diplomat said: “The significance of her breaking with Biden in the debate cannot be underestimated.”

Europeans have never been easy about US protectionism, but Trump’s increasing recourse to tariffs and other tools as the solution to nearly everything takes the friction to a different level.

He has floated a blanket tariff of between 10 and 20% on nearly all imports and of 60% or more on Chinese goods, as well as a plan to match the tariffs that other countries impose on US products. In his latest pronouncement, Trump said he would apply tariffs to any country that tried to avoid trading in dollars.

These set of policies showed how Europe’s quest for competitiveness, which was set out in a report this week by Mario Draghi, the former head of the European Central Bank, would pit Brussels and Washington against one another. No one quite knows whether Harris would be prepared to love-bomb Europe like Biden, one diplomat said.

Above all, the foreign policy part of Tuesday’s head-to-head seemed a subset of the domestic debate. So even when Harris warned that Trump was the autocrat’s patsy, she added that a defeat in Ukraine would mean Putin setting his eyes on Poland. In one of many telling lines, she asked: “Why don’t you tell 800,000 Polish-Americans here in Pennsylvania how quickly you would give [Poland] up for a favour and for what you think is friendship with a dictator who would eat you for dinner?”