Most of them were Kosovar Albanians killed by Serbian forces, often in mass killings for which few Serbs have been held accountable. But more than 2,000 Serbs, Kosovar Albanians and Roma were killed either by NATO bombs or by Kosovar guerrilla groups, like the Kosovo Liberation Army, which Mr. Thaci helped lead.
Mr. Thaci’s office declined to comment on the charges, which include murder, persecution and torture, but has historically denied similar claims. Another politician named in the indictment — Kadri Veseli, who leads a party founded by Mr. Thaci — issued a denial on Wednesday.
The postponement of the summit caps an eventful few months for Mr. Grenell and his career in foreign policy.
Between February and May, he was President Trump’s acting director of national intelligence, serving concurrently as American ambassador to Germany, a role he gave up in May, and also as special presidential envoy for Serbian-Kosovar peace negotiations, a role he has kept.
During February and March, he became an increasingly divisive figure in Kosovo because he put severe pressure on Kosovar politicians while making few demands of Serbia, an ally of Russia.
Most controversially, he was perceived to have taken sides in a political dispute between Mr. Thaci and Albin Kurti, then the prime minister. After Mr. Grenell spoke out against Mr. Kurti, the latter was abandoned by his coalition partners, leading to the collapse of his government, and causing a constitutional crisis in Kosovo.
In May, Mr. Grenell was criticized for politicizing the office of the director of national intelligence when he declassified documents related to Mr. Trump’s former national security adviser, Michael T. Flynn.
And this month, he drew bipartisan concern after leading the push to withdraw thousands of American troops from Germany, a move critics saw as detrimental to American military planning.