Bulgaria, a kingdom of south-eastern Europe, situated in the north-east of the Balkan Peninsula, and on the Black Sea. From 1878 until the 5th of October 1908, Bulgaria was an autonomous and tributary principality, under the suzerainty of the sultan of Turkey. The area of the kingdom amounts to 37,240 sq. m., and comprises the territories between the Balkan chain and the river Danube; the province of Eastern Rumelia, lying south of the Balkans; and the western highlands of Kiustendil, Samakov, Sofia and Trn. Bulgaria is bounded on the N. by the Danube, from its confluence with the Timok to the eastern suburbs of Silistria whence a line, forming the Rumanian frontier, is drawn to a point on the Black Sea coast 10 m. S. of Mangalia. On the E. it is washed by the Black Sea; on the S. the Turkish frontier, starting from a point on the coast about 12 m. S. of Sozopolis, runs in a south-westerly direction, crossing the river Maritza at Mustafa Pasha, and reaching the Arda at Adakali. The line laid down by the Berlin Treaty (1878) ascended the Arda to Ishiklar, thence following the crest of Rhodope to the westwards, but the cantons of Krjali and Rupchus included in this boundary were restored to Turkey in 1886. The present frontier, passing to the north of these districts, reaches the watershed of Rhodope a little north of the Dospat valley, and then follows the crest of the Rilska Planina to the summit of Tchrni Vrkh, where the Servian, Turkish and Bulgarian territories meet.
From this point the western or Servian frontier passes northwards, leaving Trn to the east and Pirot to the west, reaching the Timok near Kula, and following the course of that river to its junction with the Danube. The Berlin Treaty boundary was far from corresponding with the ethnological limits of the Bulgarian race, which were more accurately defined by the abrogated treaty of San Stefano (see below, under History). A considerable portion of Macedonia, the districts of Pirot and Vranya belonging to Servia, the northern half of the vilayet of Adrianople, and large tracts of the Dobrudja, are, according to the best and most impartial authorities, mainly inhabited by a Bulgarian population.
The most striking physical features are two mountain-chains; the Balkans, which run east and west through the heart of the country; and Rhodope, which, for a considerable distance, forms its southern boundary. The Balkans constitute the southern half of the great semicircular range known as the anti-Dacian system, of which the Carpathians form the northern portion. This great chain is sundered at the Iron Gates by the passage of the Danube; its two component parts present many points of resemblance in their aspect and outline, geological formation and flora. The Balkans (ancient Haemus) run almost parallel to the Danube, the mean interval being 60 m.; the summits are, as a rule, rounded, and the slopes gentle. The culminating points are in the centre of the range: Yumrukchál (7835 ft.), Maragudúk (7808 ft.), and Kadimlía (7464 ft.). The Balkans are known to the people of the country as the Stara Planina or "Old Mountain," the adjective denoting their greater size as compared with that of the adjacent ranges: "Balkán" is not a distinctive term, being applied by the Bulgarians, as well as the Turks, to all mountains. Closely parallel, on the south, are the minor ranges of the Sredna Gora or "Middle Mountains" (highest summit 5167 ft.) and the Karaja Dagh, enclosing respectively the sheltered valleys of Karlovo and Kazanlyk. At its eastern extremity the Balkan chain divides into three ridges, the central terminating in the Black Sea at Cape Eminé ("Haemus"), the northern forming the watershed between the tributaries of the Danube and the rivers falling directly into the Black Sea. The Rhodope, or southern group, is altogether distinct from the Balkans, with which, however, it is connected by the Malka Planina and the Ikhtiman hills, respectively west and east of Sofia; it may be regarded as a continuation of the great Alpine system which traverses the Peninsula from the Dinaric Alps and the Shar Planina on the west to the Shabkhana Dagh near the Aegean coast; its sharper outlines and pine-clad steeps reproduce the scenery of the Alps rather than that of the Balkans. The imposing summit of Musallá (9631 ft.), next to Olympus, the highest in the Peninsula, forms the centre-point of the group; it stands within the Bulgarian frontier at the head of the Mesta valley, on either side of which the Perin Dagh and the Despoto Dagh descend south and south-east respectively towards the Aegean. The chain of Rhodope proper radiates to the east; owing to the retrocession of territory already mentioned, its central ridge no longer completely coincides with the Bulgarian boundary, but two of its principal summits, Sytké (7179 ft.) and Karlyk (6828 ft.), are within the frontier.
From Musallá in a westerly direction extends the majestic range of the Rilska Planina, enclosing in a picturesque valley the celebrated monastery of Rila; many summits of this chain attain 7000 ft. Farther west, beyond the Struma valley, is the Osogovska Planina, culminating in Ruyen (7392 ft.). To the north of the Rilska Planina the almost isolated mass of Vitosha (7517 ft.) overhangs Sofia. Snow and ice remain in the sheltered crevices of Rhodope and the Balkans throughout the summer. The fertile slope trending northwards from the Balkans to the Danube is for the most part gradual and broken by hills; the eastern portion known as the Delí Orman, or "Wild Wood," is covered by forest, and thinly inhabited. The abrupt and sometimes precipitous character of the Bulgarian bank of the Danube contrasts with the swampy lowlands and lagoons of the Rumanian side. Northern Bulgaria is watered by the Lom, Ogust, Iskr, Vid, Osem, Yantra and Eastern Lom, all, except the Iskr, rising in the Balkans, and all flowing into the Danube. The channels of these rivers are deeply furrowed and the fall is rapid; irrigation is consequently difficult and navigation impossible.
The course of the Iskr is remarkable: rising in the Rilska Planina, the river descends into the basin of Samakov, passing thence through a serpentine defile into the plateau of Sofia, where in ancient times it formed a lake; it now forces its way through the Balkans by the picturesque gorge of Iskretz. Somewhat similarly the Deli, or "Wild," Kamchik breaks the central chain of the Balkans near their eastern extremity and, uniting with the Great Kamchik, falls into the Black Sea. The Maritza, the ancient Hebrus, springs from the slopes of Musallá, and, with its tributaries, the Tunja and Arda, waters the wide plain of Eastern Rumelia. The Struma (ancient and modern Greek Strymon) drains the valley of Kiustendil, and, like the Maritza, flows into the Aegean. The elevated basins of Samakov (lowest altitude 3050 ft.), Trn (2525 ft.), Breznik (2460 ft.), Radomir (2065 ft.), Sofia (1640 ft.), and Kiustendil (1540 ft.), are a peculiar feature of the western highlands.