Turkey says the air and ground operation dubbed "Euphrates Shield" will clear jihadists from the Syrian town of Jarabulus, which lies directly opposite the Turkish town of Karkamis.
The operation was launched just days after Ankara appeared to soften its often-confrontational line on Syrian President Bashar al-Assad, whom Turkey wants to see removed.
- Why is Turkey acting now? -
The military action began after 54 people were killed in a weekend suicide attack in the city of Gaziantep near the Syrian border that was blamed on IS.
There has also been a recent upsurge in shelling of Turkish territory from Syrian towns under IS control.
"ISIS (IS) has been directly attacking Turkish soil from Syria and so this operation is firstly a retaliation to that," Gulnur Aybet, director of Centre of Security Studies at Bahcesehir University in Istanbul (BAUCESS).
"There is a chance these attacks from ISIS will intensify and that's why Turkey thought it's necessary to take these steps, to go across the border, with tanks and ground troops," she added.
London-based Turkey analyst at Stroz Friedberg risk consultancy Emre Tuncalp agreed, saying there had been a "deterioration" in the security situation on the border.
"The Gaziantep attack and the Karkamis shelling yesterday was like the last straw."
- What is the purpose? -
However Aybet said the operation's aims were two-fold -- to counter the IS threat but also to prevent Syrian Kurdish groups moving into areas liberated from IS.
Turkey views the Kurdish People's Protection Units (YPG) militia as an extension of the outlawed Kurdistan Workers' Party (PKK), which it has denounced as a terror organisation along with the EU and the US.
The Syrian Kurds "already occupy a large strip of that border but there is this part in the middle that is still held by ISIS.
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"So the concern is: do the YPG move into areas liberated by ISIS? In moving in this way, they (Turkish forces) are also preventing the YPG moving into those areas," the professor said.
Tuncalp said Turkey had long declared its red line for the advance of the Syrian Kurdish group was the west of the Euphrates River.
He said Ankara had choked on seeing the Syrian Kurds take from IS the strategic town of Manbij south of Jarabulus and well west of the Euphrates earlier this month.
"That was really unacceptable for Turkey and was additional motivation to get more involved in that part of Syria."
- What does this mean for Syrian Kurds? -
Aybet said the military operation showed Turkey would not allow the YPG to put down roots on its doorstep and allow a fragmentation of the Syrian state.
"This has disrupted the YPG's plans. They thought the Americans would back them in the territorial conquest until the end but the Americans are playing a pragmatic game on the ground."
A US official on Wednesday said Kurdish-dominated forces had stopped moving north towards Jarabulus. "So I think we have put a lid on the Turks' biggest concern," the official said.
- Is Turkey softening on Assad? -
At the weekend, Prime Minister Binali Yildirim for the first time acknowledged that Assad was one of the "actors" in Syria, saying he may need to remain as part of any transition.
Turkey is also working more closely with Iran and Russia, Assad's last remaining major allies. So far, no world power has objected to the Turkish operation, which began just hours before US Vice President Joe Biden arrived in Ankara.
There have also been signs of a less confrontational Turkish foreign policy since Yildirim took over from Ahmet Davutoglu as premier in May.
For Soner Cagaptay, director of the Turkish Research Programme at The Washington Institute, stopping Kurdish advances in the north was now Ankara's primary goal in Syria rather than Assad's removal.
"Following the ouster of Ahmet Davutoglu, the architect of Turkey's foreign policy in the last decade, Ankara has recalibrated its Syria policy.
"Blocking PYD Kurdish advances in Syria, previously Ankara’s secondary goal, now trumps Turkey's erstwhile policy of ousting the Assad regime."