'If you want peace," runs the Latin expression, "prepare for war." It is an idea Yemen's president Abdrabbu Mansur Hadi, as a career military man, ought to know well. In the modern context of Yemen it applies most to the "southern question" - again, a topic the president should know well, as both a southerner and a long-time vice-president.
And yet Mr Hadi appears not to take the political war brewing in the south seriously enough. Rightly, the president has decided the current interests of the country are best served by keeping Yemen united. But he has not yet accepted that many others see it differently, and thus does not seem to be prepared to fight the political war for the south that is looming.
For a politician hailed as a strategist, this oversight is a strategic error. If Mr Hadi wants Yemen to remain whole, he will have to take the possibility of secession seriously.
The idea of a separate southern state in Yemen has gained popularity in the past half decade but especially so since the ousting of Mr Hadi's former boss, Ali Abdullah Saleh.
It remains more an idea than a policy, but in a way that provides it with greater raw animating power.
The grievances of southern Yemen go back a long way, but were entrenched by the unhappy union with the north in the early 1990s.
Since then, southerners have been removed from positions in the military and the public sector and many saw their lands confiscated and their businesses shut down. The Southern Movement, known as Hirak, was set up in 2007 to address some limited grievances but has since gained rapid support all across the south, centred around the city of Aden.
What happens to the southern issue remains the biggest question that the on-going National Dialogue will have to answer. Sensing this, Mr Hadi offered the Hirak movement a large share of the seats in the dialogue, only to be rebuffed by those factions that would rather discuss separation.
Although a small delegation from Hirak is taking part in the dialogue, notable factions - including one of the largest, led by the former president of South Yemen Ali Salem Al Beidh - are not, refusing to sit down with representatives of what is now increasingly called the "northern occupation".
Yet Mr Hadi acts as if the representation of this one faction means that Hirak as a whole - and, more importantly, the public opinion that animates it - is represented at the dialogue. That could prove a dangerous miscalculation.
A separate southern Yemen would be a serious change to the map of the Arabian Peninsula. If the new country followed the contours of its predecessor, formerly known as the People's Democratic Republic of Yemen, it would border both Oman and Saudi Arabia, sit on the strategically vital Gulf of Aden and include the country's largest port, the lion's share of its coastline, and most of the remaining oil reserves.
Due to that complexity alone - which would, naturally, have to be resolved in tandem with the other environmental and demographic challenges that face the entire country - it is not something Sanaa, Riyadh or most of the international community want to contemplate.
But events in the south could overtake the politicians. Although the leaders of Hirak are divided on the best political strategy, among a wide swathe of southerners there is broad agreement on issues.
The three R's continue to animate the movement: genuine political representation (whether in an inclusive government in one country or as a separate south), reinstatement of lands and jobs, and redress for what southerners say was institutionalised discrimination.
At the end of last year, Mr Hadi set up a tribunal to address some of the issues of reinstatement, promising to return to southerners property seized after the 1994 civil war. But it has moved slowly, while other events - such as drone strikes by the United States - continue to inflame the south.
By not making a genuine effort to fix legitimate grievances of southerners, Mr Hadi is making his task of steering Yemen through this political transition harder.
Hirak has distanced itself from the jihadi groups that periodically spring up across the south and from any external influence from Iran. The movement could be an ally to Mr Hadi, if only he would make them an offer.
What might that be? The best solution now on offer to Mr Hadi would mean a "grand bargain" of sorts with the south, offering some level of autonomy, perhaps, in return for dropping secessionist demands.
Whether Mr Hadi can make such an offer - and whether he can find anyone in the south to listen, agree and deliver the deal - remains an open question.
But Mr Hadi is running out of time. Whereas at the beginning of the year, he might have found factions within Hirak open to dialogue on greater inclusion in a Sanaa government, now public sentiment is hardening, moving towards autonomy and more.
The longer he waits to speak to the south about remaining in a genuine Yemeni union, the less likely he is to find anyone willing to listen.
On Twitter: @FaisalAlYafai