Trade war : Trump declares $11B tariff on EU imports. EU declares retaliatory measures.
The thing to note here is that US influence over global commerce and politics is rapidly decreasing over these measures. Other countries respected US because of its economic impact. But when you cut all these ties and take a stand against world trade as what Trump is doing now, you lose this influence.
That's where the world is leading to, right now. A weakened top leader. Which is a ripe time for someone from the second line to kick him out trying to take his place. Historically wars started like this, whenever the leader got weak and created a power imbalance.