As Estonia was repeatedly invaded and occupied during World War II, Keres was forced by the circumstances to represent the Soviet Union (1940–41, 1944–75) and Nazi Germany (1941–44) in international tournaments.

Due to these and other strong results, many chess historians consider Keres one of the greatest "Super grandmasters"[a] in history, and, along with Viktor Korchnoi, the strongest player never to become world champion.

Widely considered an Estonian national hero,[1] he was nicknamed "Paul the Second", "The Eternal Second", and "The Crown Prince of Chess".

[6] Keres achieved a very good result at the age of 17 in a Master tournament in Tallinn 1933 with 5/7 (+5−2=0), tied 3rd–4th, half a point behind joint winners Paul Felix Schmidt and V.

[8] Keres played on top board for Estonia in the 6th Chess Olympiad at Warsaw 1935, and was regarded as the new star, admired for his dashing style.

Keres' first major international success against top-level competition came at Bad Nauheim 1936, where he tied for first with Alexander Alekhine at 6½/9 (+4−0=5).

This successful string earned him an invitation to the tournament at Semmering–Baden 1937, which he won with 9/14 (+6−2=6), ahead of Fine, José Raúl Capablanca, Reshevsky, and Erich Eliskases.

[11] In 1938, he tied with Fine for first, with 8½/14, in the all-star AVRO tournament, held in various cities in the Netherlands, ahead of chess legends Mikhail Botvinnik, Max Euwe, Reshevsky, Alekhine, Capablanca and Flohr.

[9] During World War II, the previously independent nation of Estonia was invaded and occupied by the Stalinist Soviet Union in June 1940.

Botvinnik won the event, one of the strongest ever organized, with 13½/20, and Keres placed second with 11, ahead of Vasily Smyslov, Isaac Boleslavsky, Andor Lilienthal, and Igor Bondarevsky.

[9] As resident of German-occupied Estonia (then part of Reichskommissariat Ostland) in 1941–1944, Keres was able to travel across the European continent and participate in a number of international tournaments, held both in neutral countries (1944 Madrid, Lidköping, Stockholm) and in countries under Nazi German control (1942 Tallinn, Salzburg, Munich; 1943 Prague, Posen, Salzburg, Tallinn).

Before the Soviet Union reoccupied Estonia, in the autumn of 1944 Keres made an unsuccessful attempt to escape from the country over the sea to the west.

[14] Keres managed to avoid deportation or any worse fate (e.g., that of Vladimirs Petrovs); however, he may have been held in detention; precise details are difficult to ascertain.

[15] But his return to the international chess scene was delayed, in spite of his excellent form; he won at Riga 1944/45 (Baltic Championship) (10½/11).

He then won at Tbilisi 1946 (hors concours in the Georgian Championship) with a near-perfect score of 18/19, ahead of Vladas Mikėnas and a 16-year-old Tigran Petrosian.

Held jointly in The Hague and Moscow, the tournament was limited to five participants: Mikhail Botvinnik, Vasily Smyslov, Paul Keres, Samuel Reshevsky, and Max Euwe.

In his individual match with the winner, Botvinnik, he lost four of five games, winning only in the last round when the tournament's result was already determined.

Then in 1951, he triumphed again at Moscow, URS-ch19, with 12/17 (+9−2=6),[9] against a super-class field which included Efim Geller, Petrosian, Smyslov, Botvinnik, Yuri Averbakh, David Bronstein, Mark Taimanov, Lev Aronin, Salo Flohr, Igor Bondarevsky, and Alexander Kotov.

Keres won Pärnu 1947 with 9½/13 (+7−1=5), Szczawno-Zdrój 1950 with 14½/19 (+11−1=7), and Budapest 1952 with 12½/17 (+10−2=5),[9] the latter ahead of world champion Botvinnik and an all-star field which included Geller, Smyslov, Gideon Ståhlberg, László Szabo, and Petrosian.

Altogether, in 11 Olympiads, playing for both the USSR and Estonia (counting the unofficial Munich 1936 event), and in 161 games, Keres accumulated a brilliant total of (+97−13=51), for 76.1%.

His detailed Euroteams results are:[21] Keres also represented the USSR in many international team matches, in Europe and the Americas, with great success.

[22] Beginning with the Pärnu 1947 tournament, Keres made some significant contributions as a chess organizer in Estonia; this is an often overlooked aspect of his career.

Also in 1970, Keres's 3:1 win over Ivkov on the tenth board gave victory to the Soviet team in the match vs Rest of the World.

That same year, he made his last Soviet Championship appearance, at Moscow for URS-ch41, scoring 8/17 for a shared 9–12th place, as Spassky won.

Over 100,000 were in attendance at Keres' funeral in Tallinn, Estonia; and FIDE President Max Euwe, his old friend and rival, was also present.

There are also the annual Keres Memorial tournament held in Vancouver[27] and a number of chess clubs and festivals named after him.

[28] The unofficial Chessmetrics system places Keres in the top 10 players in the world between approximately 1936 and 1965, and overall he had one of the highest winning percentages of all grandmasters in history.

Perhaps best-known is the Keres Attack against the Scheveningen Variation of the Sicilian Defence (1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e6 6.g4), which was successfully introduced against Efim Bogolyubov at Salzburg 1943, and remains an important line.

An original system on the Black side of the Closed Ruy Lopez (1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.0-0 Be7 6.Re1 b5 7.Bb3 d6 8.c3 0-0 9.h3 Na5 10.Bc2 c5 11.d4 Nd7) was introduced by Keres at the 1962 Candidates' tournament, and was popular for several years.

His rival Samuel Reshevsky said that Keres failed to become world champion because he lacked a killer instinct and "was too mild a person to give his all in order to defeat his opponents.