In the north, Croatia often clashed with the Principality of Hungary; the state retained its borders and, to some extent, expanded with the disintegrated Lower Pannonia.

Tomislav attended the 925 Council of Split, convened by Pope John X, to discuss the use of Slavic languages in liturgy, and ecclesiastical jurisdiction over both Croatia and the Byzantine Theme of Dalmatia.

[5] The Chronicle of the Priest of Duklja mentions that Tomislav, the length of whose rule was specified as 13 years, successfully fought the Battle of Drava River with the Hungarians.

Since the Venetian chronicler Andrea Dandolo and a notary of King Béla III mention Hungarian victories against Croatia in the same period, however, both sides had gains.

[3] During the early 10th century, Croatia was divided into 11 counties: Livno, Cetina, Imotski, Pliva, Pset, Primorje, Bribir, Nona, Knin, Sidraga, and Nin.

[8] Byzantine emperor and chronicler Constantine VII writes in De Administrando Imperio that at its peak, Croatia could have raised a military force composed of 100,000 infantry, 60,000 horsemen, and a naval fleet of 80 large ships and 100 smaller vessels.

[3] According to palaeographic analysis of the manuscript of De Administrando Imperio, the population of medieval Croatia was estimated at 440,000 to 880,000; its military force probably consisted of 20,000–100,000 infantrymen and 3,000–24,000 horsemen organized into 60 allagia.

[15] A note preceding the proceedings of the 925 Council of Split calls Tomislav king "in the province of the Croats and in the Dalmatian regions" (in prouintia Croatorum et Dalmatiarum finibus Tamisclao rege).

[20][21] In 925, Pope John X convened a church council in Split to decide which bishops in the former Roman province of Dalmatia would have ecclesiastical jurisdiction.

Before the council, Bishop Gregory was responsible for a significantly larger territory than Archbishop John; however, his reputation and finances could not compete with that of the Archbishopric of Split.

[29] Tomislav may have received some control of the Theme of Dalmatia's coastal cities or a share of collected taxes for his assistance to the Byzantine Empire.

[14] Since Croatia was harboring Bulgarian enemies and was allied with the Byzantine Empire, Simeon attacked with an army led by Duke Alogobotur.

[38] Josip Lučić and Franjo Šanjek's 1993 Hrvatski povijesni zemljovid (Croatian Historical Map) depicted the extent of Tomislav's kingdom.

South of it, the king held "modern Croatia, Slavonia, northern and western Bosnia, and the territory along the Dalmatian coast from what is now Rijeka to at least the mouth of the Cetina River (excluding the scattered Byzantine towns)".

[3] Fine criticized the relationship between Tomislav's territory and modern Croatian nationalist sentiment in his 2006 book, calling 10th-century sources unreliable and "roughly a third" of Croatia's perceived eastern land "entirely speculation".

[43] Fine wrote, "It is possible that Croatia really did have some of it, but Bulgaria may have had some of it; early Serb entities may have had some of it, not to speak of various župans and other local Slavic lords who in any serious way answered to no one.

He criticised Lučić and Šanjek's delineation of Tomislav's eastern border as "nationalist map-making" and "distorting the perceptions of children on their nation's history in a way that promotes interpreting later events as territorial loss and fragmentation.

In Zagreb, the Croatian capital, a square named Tomislav was dedicated in November 1927; a monument by sculptor Robert Frangeš-Mihanović was erected that year.

Tomislav's statue in Zagreb was depicted on the reverse of the Croatian 1000 kn banknote issued in 1994,[44] and his name is used for a dark beer brewed in Croatia.