A just war, as the world understands it, must truly be just and have a real justification to start a war against another country. It is usually when there is a real threat that another country will take over your country or cause real harm to your people. In this sense of self-defense, you may defend yourself, but it should not be interpreted broadly to justify attacking another country because of possible threats or possible disadvantages that country might bring.
These threats must be very clearly formulated as a real danger to the people, or as a substantial diminishing of the security of the population, or a substantial impoverishment or damage to the economy, and so on. This must be clearly defined. First, the threats must be clarified, and to what extent they exist. Sometimes this also applies to protecting those who are cruelly persecuted, for example Christians in other countries.
In such cases, another country, a Christian country, could defend these Christians. This could also be considered a kind of just war. This was the case with the so called Crusades in the Middle Ages, which defended Christians and pilgrims from attacks and cruelties at that time by Muslim armies in the East, in the Holy Land.
I would also consider that President Trump, who took action against Islamic extremist terrorist groups in Nigeria, carried out a form of just war. These groups have for many years cruelly persecuted and killed the Christian population in northern Nigeria. He did this with the consent of the government of Nigeria. Therefore, this can also be seen as a just war.
Just war also implies that all other means, diplomatic solutions, peaceful solutions, and negotiations, have been attempted beforehand. This must also be taken into consideration.