After his accession Antiochus took care to maintain good relations with the Roman Republic, sending an embassy to Rome in 173 BC with a part of the unpaid indemnity still owed from the 188 BC Treaty of Apamea.
While there the embassy secured a renewed treaty of friendship and alliance with Rome, greatly helped by the fact Antiochus had come to power with the help of Eumenes II, Rome's principal ally in the region.

The guardians of King Ptolemy VI Philometor demanded the return of Coele-Syria in 170 BC, declaring war on the Seleucids on the assumption that the kingdom was divided after Antiochus' murder of his nephew.
However Antiochus had prepared more thoroughly and launched a preemptive strike against Egypt, defeating a Ptolemaic force and seizing Pelusium.
He then advanced into Egypt proper, conquering all but Alexandria and capturing King Ptolemy.
This was partially achieved because Rome (Ptolemaic Egypt's traditional ally) was embroiled in the Third Macedonian War and was not willing to become involved elsewhere.

To avoid alarming Rome, Antiochus allowed Ptolemy VI to continue ruling as a puppet king from Memphis.
Upon Antiochus' withdrawal, the city of Alexandria chose a new king, one of Ptolemy's brothers, also named Ptolemy (VIII Euergetes).
The Ptolemy brothers agreed to rule Egypt jointly instead of fighting a civil war.

In 168 BC, Antiochus led a second attack on Egypt and also sent a fleet to capture Cyprus.
Before he reached Alexandria, his path was blocked by a single elderly Roman ambassador named Gaius Popillius Laenas who delivered a message from the Roman Senate directing Antiochus to withdraw his armies from Egypt and Cyprus or consider himself in a state of war with the Roman Republic.
 Antiochus said he would discuss it with his council, whereupon the Roman envoy drew a line in the sand around Antiochus and said: "Before you leave this circle, give me a reply that I can take back to the Roman Senate."
This implied Rome would declare war if the King stepped out of the circle without committing to leave Egypt immediately.
Weighing his options, Antiochus decided to withdraw.
Only then did Popillius agree to shake hands with him.
After his accession Antiochus took care to maintain good relations with the Roman Republic, sending an embassy to Rome in 173 BC with a part of the unpaid indemnity still owed from the 188 BC Treaty of Apamea. While there the embassy secured a renewed treaty of friendship and alliance with Rome, greatly helped by the fact Antiochus had come to power with the help of Eumenes II, Rome's principal ally in the region.
The guardians of King Ptolemy VI Philometor demanded the return of Coele-Syria in 170 BC, declaring war on the Seleucids on the assumption that the kingdom was divided after Antiochus' murder of his nephew. However Antiochus had prepared more thoroughly and launched a preemptive strike against Egypt, defeating a Ptolemaic force and seizing Pelusium. He then advanced into Egypt proper, conquering all but Alexandria and capturing King Ptolemy. This was partially achieved because Rome (Ptolemaic Egypt's traditional ally) was embroiled in the Third Macedonian War and was not willing to become involved elsewhere.
To avoid alarming Rome, Antiochus allowed Ptolemy VI to continue ruling as a puppet king from Memphis. Upon Antiochus' withdrawal, the city of Alexandria chose a new king, one of Ptolemy's brothers, also named Ptolemy (VIII Euergetes). The Ptolemy brothers agreed to rule Egypt jointly instead of fighting a civil war.
In 168 BC, Antiochus led a second attack on Egypt and also sent a fleet to capture Cyprus. Before he reached Alexandria, his path was blocked by a single elderly Roman ambassador named Gaius Popillius Laenas who delivered a message from the Roman Senate directing Antiochus to withdraw his armies from Egypt and Cyprus or consider himself in a state of war with the Roman Republic.  Antiochus said he would discuss it with his council, whereupon the Roman envoy drew a line in the sand around Antiochus and said: "Before you leave this circle, give me a reply that I can take back to the Roman Senate." This implied Rome would declare war if the King stepped out of the circle without committing to leave Egypt immediately. Weighing his options, Antiochus decided to withdraw. Only then did Popillius agree to shake hands with him.
Antiochus IV Epiphanes