The savaging of Poland by the Mongols since 1241/42 never took place in TTL. Thus, the competent prince of Silesia, Heinrich / Henryk the Pious managed not only to live longer, but also getting crowned king Henryk II of Poland, first one since 1079. Although many Piast princes continued to reign in other parts of Poland, his family could keep the king's title. In 1293, Poland attacked the Russian princedom of Halicz-Volhyn, taking about one third of the latter's territory.
But now, Poland felt increasingly threatened by the Germans and the Przemyslids. The king tried to fight them in 1301 to break free, but this only lead to a coalition of the Teutonic Order, Bohemia, Moravia and the HRE under king Otto IV of Brandenburg formed against him. The war between Germans and Poles ended in 1308, not to the latter's favor. The Teutonic Order acquires Pommerellen (OTL West Prussia, the Poles also call it Eastern Pomerania), Silesia became a German fief; the western third went to Brandenburg, which earlier had acquired the Lower Lausitz / Lusatia, the rest was divided between the two Przemyslid lands, Bohemia and Moravia. As a consequence, king Boleslaw V was toppled by discontent nobles, who elected Kazimierz III new king, who made an "everlasting alliance" with the (in many ways still independent) rulers of Mazovia. In 1363, when the Anjou dynasty in Hungary died out, the nobles there elected Wladyslaw IV of Poland new king. But in 1371 already, this influence ended with the death of incompetent king Kazimierz IV.
Rise to power
In 1394/95, the Black Death hit Central Europe. Although Poland was also affected, by imposing a quarantine it was mostly saved and didn't have to pay such a high toll as its neighbors. Many persecuted Jews fled to Poland. This development proved to be a great advantage: In 1404 Poland attacked the Teutonic Order, won the first Teutonic-Polish war; Pommerellen became Polish again, thus gaining an important access to the sea. 1426, Boleslaw VI of Poland and Birger II Eriksson allied against the Teutonic Order. 1432 they were victorious again; Poland kept its conquests from the last war and also gets Wolhynien, Sweden got Estonia (that's only the northern half of OTL Estonia, though). In another war 1432-35, Poland conquered the princedom of Smolensk.
In the 2nd half of the 15th century however, the power of the Polish kings shrunk, and in 1470 the young king had to accept a new constitution, which gave every noble (15% of the Polish population!) the right to vote in the parliament, and introduced separation of powers. 1472-76, the Polish-Bohemian War took place; after the death of king Vaclav IV the Mad, the Poles had hoped for an easy victory, but the new king Jan II lead the Czech armies surprisingly well and drove the Poles back. OTOH, in 1503 the Slovakian estates made an alliance with Poland, after Hungary was conquered by the Rum-Seljuks.
King Boleslaw VII and his successor Boleslaw VIII now started a policy with the aim of strengthening the power of Poland and themselves: They built many printing presses and several universities, founded colonies (in 1539, Poland took OTL Trinidad and Tobago; and when in 1544 in Haraldsborg, the (almost) last colony of Denmark, chaos ensued after the mother country had fallen into civil war, Poland sent soldiers to the city, occupying it) and reformed Poland in various ways. 1533-35, they fought off the Russians in the former princedom of Smolensk who had asked the Grand Prince for help against Poland. In 1538, Poland secularized church property, which helped them gain money for the reforms.
The elected kings
In the following half century, the Polish nobles elected various kings of other countries, the most famous among them Alasdair IV and Henry V. This, so they thought, was the best way to be defended against the Russians. However, when Polish king Ludwik felt angered by the new Czar and wanted to get rid of the Russian threat, started a preventive war against Vladimir-Suzdal in 1604. But the Polish army was destroyed in the battle of Kostroma 1610, and in the Peace of Thurau 1612, Poland had to cede the former princedom of Smolensk back to Vladimir. Finally, in the English-Polish War 1613/14 Poland lost its colonies of OTL Trinidad, Haraldsborg to England.
When François IV rose in Europe, they joined his side. 1637, Poland joined the HREGN, to be better protected against the Russians, and even got one electoral vote. (Its German neighbors secretly claimed that this wes for keeping them in check.) Thus, they also fought on France's side in the anti-French War; but 1686, Vladimir-Suzdal entered the war, although fighting only against Poland. In the peace of Minsk 1690, Poland ceded the former Prussian province Wolhynien (the old Russian princedoms Turov-Pinsk and parts of Halicz-Volhyn) to Vladimir, which left the war. And now, the Baltic League installed Jakub of Sulkowski as Polish anti-king, which threw Poland into Civil War. In the peace of Amsterdam, he was vindicated.
1715, Poland entered the war between Vladimir and Novorossiya, won Wolhynien back.
In 1762, the very capable Stanislaw III became king. 1764, he had the first telegraph in Poland built. 1772, he was elected Holy Roman Emperor, the first Pole to achieve this. He lost the title in 1784 when the HRE was dissolved officially, but 1788, after the heirless death of Kristian V, last Welf king of Denmark-Braunschweig, he started the regency for said country.
However, under his reign Poland also fought for the cause of monarchy in the French Republican Wars, so he lost this country too in the peace of Frankfurt 1793.
Things went down further: 1806, Novorossiya and South Russia attacked Poland, took the former Russian lands back. Poland east of the Vistula stayed occupied. After the Poles rose up against Russian occupation 1838/39, Novorossiya attacked and conquered all of Poland. The last king fled to Sweden, later to Britain.