If the new state was to expand by conquest, its line of advance was already foreshadowed.  For the present, it could hardly break back into Asia Minor, occupied as this was by Moslem principalities sanctioned by the same tradition as itself, namely, the prestige of the Seljuks.  To attack these would be to sin against Islam.  But in front lay a rich but weak Christian state, the centre of the civilization to which the popular element in the Osmanli society belonged.  As inevitably as the state of Nicaea had desired, won, and transferred itself to, Constantinople, so did the Osmanli state of Brusa yearn towards the same goal; and it needed no invitation from a Greek to dispose an Ottoman sultan to push over to the European shore.

Such an invitation, however, did in fact precede the first Osmanli crossing in force.  In 1345 John Cantacuzene solicited help of Orkhan against the menace of Dushan, the Serb.  Twelve years later came a second invitation.  Orkhan’s son, Suleiman, this time ferried a large army over the Hellespont, and, by taking and holding Gallipoli and Rodosto, secured a passage from continent to continent, which the Ottomans would never again let go.

Such invitations, though they neither prompted the extension of the Osmanli realm into Europe nor sensibly precipitated it, did nevertheless divert the course of the Ottoman arms and reprieve the Greek empire till Timur and his Tartars could come on the scene and, all unconsciously, secure it a further respite.  But for these diversions there is little doubt Constantinople would have passed into Ottoman hands nearly a century earlier than the historic date of its fall.  The Osmanli armies, thus led aside to make the Serbs and not the Greeks of Europe their first objective, became involved at once in a tangle of Balkan affairs from which they only extricated themselves after forty years of incessant fighting in almost every part of the peninsula except the domain of the Greek emperor.  This warfare, which in no way advanced the proper aims of the lords of Brusa and Nicaea, not only profited the Greek emperor by relieving him of concern about his land frontier but also used up strength which might have made head against the Tartars.  Constantinople then, as now, was detached from the Balkans.  The Osmanlis, had they possessed themselves of it, might well have let the latter be for a long time to come.  Instead, they had to battle, with the help now of one section of the Balkan peoples, now of another, till forced to make an end of all their feuds and treacheries by annexations after the victories of Kosovo in 1389 and Nikopolis in 1396.

Nor was this all.  They became involved also with certain peoples of the main continent of Europe, whose interests or sympathies had been affected by those long and sanguinary Balkan wars.  There was already bad blood and to spare between the Osmanlis on the one hand, and Hungarians, Poles, and Italian Venetians on the other, long before any second opportunity to attack Constantinople occurred:  and the Osmanlis were in for that age-long struggle to secure a ‘scientific frontier’ beyond the Danube, whence the Adriatic on the one flank and the Euxine on the other could be commanded, which was to make Ottoman history down to the eighteenth century and spell ruin in the end.