China sent more than a dozen warplanes across the unofficial line dividing the Taiwan Strait on Tuesday morning, a fresh squeeze on Taiwan and another sign that pressure around the island keeps building.

• Starting just before 9 a.m., 18 Chinese aircraft flew out over the water and 13 of them crossed the median line into Taiwan's northern, central and southwestern air space.
• The planes included J-10, J-11 and J-16 fighters plus a KJ-500 early-warning aircraft, working alongside Chinese navy ships.
• Beijing called the operation a "joint combat readiness patrol," a label Taiwan's defense ministry rejects as cover for harassment.
• Taiwan tracked the movements with surveillance systems and responded with its own aircraft, ships and land-based missile units.

Outlook: These crossings have become close to routine, so expect more of them — and the risk that one bad moment turns into something bigger.